If a factor is x+4, that means that the coefficient of x^2 (the number in front of it, which would be 1) and that 20 should be divisible by 20. Now, if you divide 20 by 4, you would get 5, and 1/1 = 1. So, the 2 factors you have is
(x+4)(x+5)
When you FOIL, you would get x^2+9x+20, which means
Using Pythagoras’ theorem a2 + b2 = c2 you can find out the third (longest) side. (6x6) + (8x8) = 100 (the square root of 100 is 10) so ur final sum is 6+8+10 = 24 mm
