Question 1078613
The phrase "y is directly proportional to x^2" means
{{{y=k*x^2}}} for a some constant {{{k}}} .
In that product, if you double x (you replace {{{2x}}} for {{{x}}} ),
you get {{{y=k*(2x)^2=k*2^2*x^2=k*4*x^2=4*(kx^2)}}} ,
so you quadruple the value of {{{y}}} and get {{{4a}}} .