being directly proportional means that both variables are increasing at the same rate, so both variables will be doubling. in that case:
a. y = x(^2)
if x is doubled
(2y) = (2x)^2
y is increasing by a factor of 2
b. y = x(^3)
if x is doubled
(2y) = (2x)^3
y again, increases by a factor of 2