Your 36" will become 35 1/2" after seams. 158 divided by 35 1/2 is 4 1/2, so it takes 4 1/2 widths to fit around your center panel. That means you have to cut the 18" dimension into 5 equal strips ( in order to have 4 1/2 strips to use.)
18" divided by 5 is 3.6.
Cut the long ways of your border fabric into 3 1/2" strips -- this allows just a smidgen of error, so cut very carefully!