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

I suppose it would be isoceles, then the side length would be square root of (121/2) and the area would be (121/4)