Question 213783
This can be expressed as:
{{{x = ks^2/t}}} where k is the contant of variation. 
To find out what happens to x when s is doubled, substitute s = 2s.
{{{x = k(2s)^2/t}}}
{{{x = 4ks^2/t}}} Substitute {{{ks^2/t = x}}}
{{{x = 4x}}}
X quadruples when s is doubled.
TO find out what happens to x when both s and t are doubled, substitute s = 2s and t = 2t.
{{{x = ks^2/t}}}
{{{x = k(2s)^2/2t}}}
{{{x = 4ks^2/2t}}}
{{{x = 2ks^2/t}}} Substitute {{{ks^2/t = x}}}
{{{x = 2x}}}
x is doubled when both s and t are doubled.