A= The width times the length
159 3/8= 18 6/8(I just put it into 8ths instead of fourths to make it easier) • X
Divide by 18 6/8 for both sides which will give you your X.
Meaning the 18 6/8 will cancel out on the one side. So that you can solve for that length(I would solve it but I don't have a calculator).