Question 484210
I don't know if c should be a side or the hypotenuse, 
If it should be a side:
a*2 + c^2 = b*2
7^2 + c^2 = 81
49 + c^2 = 81
c^2 = {{{sqrt(32)}}}
c = 2{{{sqrt(2)}}}
If it's a hypotenuse 
a^2 + b^2 = c^2
7^2 + 9^2 = {{{sqrt(130)}}}
c = {{{sqrt(130)}}}