Pythagoras’ theorem
a^2 + b^2 = c^2
Where a and b are two sides on a triangle and c is the hypotenuse.
Rearrange to find one to the sides that isn’t the hypotenuse.
a^2 = c^2 - b^2
a^2 = 25^2 - 10^2
a^2 = 625 - 100
a^2 = 525
(square root)
a = √525
= 5√21