Hi, Alex -
Just checking your notation: do you mean sqrt(b^2 + c^2)?
If that's the case, then 25 = sqrt(20^2 + c^2)
square both sides: 25^2 = 20^2 + c^2
625 = 400 + c^2 now subtract 400
225 = c^2 now take the square root of both sides
+15, -15 = c
If this is not the correct equation, please let me know.