The only way you have of finding the length of the missing side is to ASSUME that the lengths of all 3 of them form a Pythagorean triple. You know that you can go ahead and do this when you see the little red 'corner' drawn in one vertex of the triangle. Then you know that the angle is a right angle, the triangle is a right triangle, and the sides form a Pythagorean triple.
Then (missing side) = √( 1.7² - 1.5²)
= √(2.89 - 2.25)
= √0.64 = 0.8 .
Without that little red box corner-symbol, there would be no way. You would not stand a chance.
