Help Calculus please! find the max distance/height of projectile...?

you are on the right track by finding the derivative. the next step is to find where that derivative is equal to zero. that will tell you a "critical point" where the slope of y(x) is zero. this is either a min or max for the original function. if the derivative is zero at only one point, say at point P then you have to test a point < P and a point > P to determine if it is max or min. so pick 2 values of x A & B where A