All you do is solve for z. So 8z-13=7. To solve for a variable you have to do the opposite to cancel out the numbers on each side. First you add 13 over to 7 to cancel out 13 on the z side. So 8z=20 is what you have left. Then you divide 8 on both sides to cancel out 8 on the z side. what you are left with is z=2.5. Lastly you plug and check. 8(2.5) - 13 = 7, 20-13=7. And thats correct so z is 2.5.
You use x= -b/2a in order to get your maximum so x= -10/ 2(-5) and you get x=1. You plug 1 into the function as h(1)=-5(1)^2 + 10(1)+ 20 which gets you 25
