I am math challenged! HELP!
 

I have a quilt top that is 54 x 64. If I want to leave 3" all around the top for my quilter that makes 60 x 70. Right?
I have a backing piece that is 56 x 102.
Will this backing piece be usable? I thought you could sew along the selvage edge or the cut edge, cut on the folded part, open it up, and create a larger piece. But I forget which edge to sew!

Which edge do I sew to make the backing longer? Which edge to make it wider?

Will the backing piece I have work for the top if I sew/cut it?

TIA
dee

I'll frame the best answer I get!

First off, if you have a regular quilter, ask what they prefer. Some don't care so much about backing seams -- others have some very firm ideas.

You are correct in your math that you want 60x70" for the back.

You might want to consider the "Flynn Method", I was looking for a handy visual explanation that I know I've seen before, but I'm coming up with John's and am concerned that the math might be intimidating. There are videos but I don't know what to recommend,
https://www.flynnquilt.com/free-lessons.htm

Basically when you are a couple inches off, it can be a better use of the fabric to cut it on a diagonal and slide up the pointy edge which makes the whole piece wider (and shorter).

Thank you so much for your help!

Yes, check with your quilter as to her preferences. If you really need to have that extra width you may consider this method, and again your quilter may or may not like a diagonal join in the backing.

I have never done a diagonal seam on a quilt but I guess I will try it.
If it doesn't work out, I will have some "new" fabric to cut sashing or squares from.

Thanks to you, Gay, for your help!
This must be the way to do what I need since you echoed what IceBlossom suggested.

I always get the best answers from this site!

Merry Christmas!

I don't do diagonal seams. I would just piece the backing using a combination of horizontal and vertical seams, keeping care to keep the grain going in the same direction, especially if your fabric is directional.

if you just need it for the quilting machine, is it possible to just sew the added inches around the main backing piece since it is just going to be cut off in the end after the quilting is done anyhow.

LOVE YOUR IDEA! Backing is a solid color, and I hadn't thought of that. It's a donation quilt plus I'm helping someone adjust to a variety of fabrics, batting, and backings as she gets more "acquainted" with her long arm.

Thanks so much!

THANKS!
There's ANOTHER thought I hadn't had.
Will try it and see if it works! WON'T CUT ANYTHING YET, THOUGH.

Thanks for your help!
I think I have enough ideas to help me find a solution.

Siometimes the easiest idea is the best idea!

Might consider adding extra to both sides so it ieasier for the longarmer to center the "good part"