https://vle.mathswatch.co.uk/images/questions/question15325.png
You are just meant to use the Pythagorean theorem.
Is that enough info?
What does Pythagoras's theorem say?
It says that a^2+b^2=c^2
Well it says that if and only if a triangle is right-angled, then the square on the hypotenuse (the longest side) is equal to the sum of the squares on the other two sides
