How do you find the perimeter of triangle abc?

Alright, so we'll use the Pythagorean Theorem for this one. 
⇒ a²+b²=c² 

The hypotenuse of  a triangle is represented by c² and we're given the values for a and b.  For this triangle, we'll make 15 = a and 20 = b.  Plug this into the equation and we'll have (15)² + (20)² = c ².  This gives us 625 = c².  Solve for c by square rooting both sides of the equation.  √(625) = c.  We now know that c = 25.  To take the perimeter, we add a + b + c ⇒ (15) + (20) + (25) = Perimeter.

-Hope this helps!