3s × 3s -s = 72
(3s × 3s ) -s = 72
9s - s = 72
8s = 72
s = 72/8
s = 9
In this kind of mathematical equations, the use of the BODMAS rule is extremely helpful. Just for an over view:
B stands for Brackets first (parentheses)
O stands for OF (orders i.e. powers, square roots, cube roots etc)
DM stands for Division and Multiplication (start from left to right)
AS stands for Addition and Subtraction (start from left right)