Use the pythagorean theorem since it's a right triangle. 12 + r is the hypotenuse and r and 24 are the legs so r squared + 24 squared = (12 + r) squared. Simplify and get r squared plus 576 equals r squared + 24r +144. Subtract r squared from both sides and then subtract 144 from both sides. You're left with 432=24r. Divide both sides by 24. r=18
