You can use Pythagoras Theorem for this, where the
(adjacent side ²) + (the other adjacent side ²) = hypotenuse²
so in this question, 7 and 24 are the adjacent sides, and x is the hypotenuse, we can just fit the numbers back in the equation, which will make
7² + 24² = x² (Pythagoras theorem)
x = 25
