(24-y)/4=4
i need to solve for y but cannot remember how to execute the process

You need to isolate the variable by itself on one side of the equation.  You do so by performing the inverse operations that were done to the variable

(24-y)/4=4  multiply both sides by 4

24-y=16  subtract 24 from both sides

-y=-8  divide both sides by -1

y=8

3cm and 6 and 9 cm that a possibility of that question