Answer is the unknown side is 10
Step by step
We use Pythagorean theorem to solve this
a^2 + b^2 = c^2
The two short sides are a and b
The hypotenuse or diagonal is c
we are looking for side b (or x)
a^2 + b^2 = c^2
24^2 + b^2 = 26^2
576 + b^2 = 676
subtract 576 from both sides to isolate b
576 - 576 + b^2 = 676 - 576
b^2 = 100
√ b^2 = √100
b = 10