Find the area and perimeter of a right triangle with hypotenuse of 29cm and leg of 21cm?

It's sort of Pythagoras in reverse. The hypotenuse is given and one leg. So,
29²cm² - 21²cm² = 400cm²

400cm² is the square of the second leg. We have to extract the root:
sqrt(400cm²) = 20cm.

The perimeter is 20cm + 21cm + 29cm = 70cm.

The area is the area of the rectangle (leg1 * leg2) divided by 2 [the hypotenuse cuts the rectangle in two]:
20cm * 21cm / 2 = 210cm²