Hi
According to Pythagoras :
The square of hypotenuse is the sum if the square if the two other side
so : 7^2 + X^2 = 25^2
49 + X^2 = 625
X^2 = 625-49
X^2 = 576
this equation as two roots . As a triangle cannot have a negative side value, I will only take positive value
so X = V576 =24