"Write an expression to represent how much shorter the base is compared to the side."
It says "how much shorter the base is", so the base is shorter than the side, and the side is longer than the base.
Just to see this:
Base < Side
4x + b < ax + 9
To determine how much shorter the base is, you subtract: larger quantity - smaller quantity.
The larger quantity is the side, so the expression is
(ax + 9) - (4x + b)