x+(-a) is rational because it is the sum of two rational numbers. You can see them as fractions, if x and -a are rational numbers they can be represented by fractions and the sum of two fractions is a fraction, that is a rational number.
πx^2 + π(2x)^2 = 80π x^2 + 4x^2 = 80 5x^2 = 80 x^2 = 16 x = +/- 4 but we need the positive one because there's no negative measurement so the answers are: 4m and 8m
