A= 28. The formula for the Pythagorean theorem is A^2 + B^2 = C^2 So plug in what you know. A^2 + 21^2 = 35^2 you get A^2+ 441 = 1225. You then take away 441 from 1225 and you get A^2 = 784. You have to find the square root of 784 which is 28. So side A is 28.
2(wl+hl+hw) is the surface area of the rectangular prism If you were to double the dimensions 2(2w2l+2h2l+2h2w) This simlifies to 8(wl+hl+hw). The surface area increases by a factor of four.