Make up a right triangle 12cm, 15cm, and 20cm? 
unit 3 page 43

That may not form a right triangle. First I'll try to draw 
it as accurately to scale as possible to see whether a 
triangle with those lengths as sides looks like it could 
form a right triangle.  Here is such an accurately drawn 
triangle:



It looks like it is not quite a right triangle because
the angle at the lower right looks a little too large to
be 90° and more like about 95°.  But looking and seeing
is not considered good math, so we will have to use the
formula from the Pythagorean theorem.  If it were a
right triangle the longest side 20 cm would have to be
the hypotenuse. so we would have to have that

               c² = a² + b²
              20² = 12² + 15²
              400 = 144 + 225
              400 = 369

No that's false, so it isn't a right triangle.

Edwin