Keep your mom, you only have one. But anyway;
Use the Pythagorean Theorem: A^2 + B^2 = C^2
12^2 + 21^2 = C^2
144 + 441 = C^2
585 = C^2
square root of 585 = approximately 24.2
5x^2 + 6x +13.88 is what is is simplified hope it helped a bit
The answer is A 100 square centimeters