Slope intercept form is written as y = mx + b.

So, for this statement, the equation would be: p = 25h + 600

This means that his pay is equal to $600 plus an additional 25 dollars for each hour he works overtime.

25h+600 = what

I think that what this question is seeking is the format of the equation, with both unknown variables shown. You would need to have been supplied a value for the number of hours (h) in order to solve for the what, which is shown here as p (the total pay). Once the number of hours is known, then it can be substituted for h, and the total pay/p/what can be calculated.