Calculus

I cant math for poop, so I need your help.

How would one find the maximum area of a Right Angle Triange with a Hypotenuse of 21cm using calculus. 

Im stumped, I can do it with a rectangle but thats as far as i go.

Thanks :X

Optimization Problem

two equations:

b^2+h^2=21^2

A=0.5*b*h

sub the first into the second, and find the maxima by taking the first derivative, and setting to 0, then solve for the variable.