I need help again I used to love math but I’m not good at it no more

Even if you're not that good, you can still love it!
Well x stands for the money she had in her pocket when she first started.
38.45 is the price of a new shirt.
17.71 is what's left in her pocket after spending the price on the shirt.
So that means you have to subtract 38.45 from "x" to get 17.71:
x - 38.45 = 17.71