Solution :
Let the third side be = x
Case 1. Find it if it's the hypotenuse.
Solution : Pythagoras Theorem = (H)²=(P)²+(B)²
= On substituting the values = (n)²= 4²+3²
= x²=16+9
= x²= 25
= x=√25
= x = 5
Case 2 . Find it if it's one of the legs
= If it is one of the legs , then the longest side of them will be the hypotenuse which is 4.
Legs = 3,n
Hypotenuse = 4
Pythagoras Theorem = (P)²+(B)²=(H)²
= 3²+x²=4²
= 9+x²= 16
= x²= 16-9
= x²= 7
= x= √7
= x = 2.64 (approx.)