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