Lol I didnt even think it was -b/2a. That didnt come to mind at all xD
I differentiated to find the stationary point, found out what x was, and then y at the peak. I also found out the y value of P, by subbing in x = 30 into the equation. then...
.:. d = (y value of peak) - (y value of P)
=7.2...