Cut the yellow fabric in half along the fold. Call this fabric A
Cut the other fabric in half along the fold. Call this fabric B.
Sew two SHORT sides of fabric B together to make one long 22" wide (or so) section.
Sew one A half to the long B section. Press. (The lengths may not match but that's okay.)
Sew the second half of A to the other side of the B section.
The finished pieced back will look like this. Trim as needed. IF your backing fabrics were at least 43" wide, you will have a pieced backing that is approximately 63" wide, barely enough extra to make it easy for quilting.
Jan in VA