Question

Help pls!!

Answer 1

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.