Why is it not possible to from a right triangle with the lengths 2,4 and 7?

It is not possible to form *any* triangle from the lengths 2, 4, and 7. The two short sides don't add to a length at least as long as the long side.

3. The missing side is given by the Pythagorean theorem:
  √((6 in)^2 + (12 in)^2) = 6√5 in ≈ 13.416 in

a is 9 and b is 4

Step-by-step explanation:

When you subtract a-b, a is 9 and b is 4 which makes it 5. And ab, is 9 x 4 which equals 36.