Answer:
58
The real answer would be the root of 58, but I am assuming that you want the 58 because it already provides the root symbol on the question mark.
Explanation:
Pythagorean Theorem is a^2 + b^2 = x^2 and was invented by the Greek guy named Pythagoras which opened up many branches of math.
Just plug in 7 in a or b and 3 as the other variable of a or b.
Then you solve it by 7^2 + 3^2 which is 49 + 9 which is 58.
BUT YOU AREN'T DONE....
since you squared the a's and b's you need to square the c so you need to take the root of 58.
Since the diagram seems to already provide it, it can be 58.
Hope this helps!