when y = 21, x = 3
when y and x vary directly, that means there is something that you multipy x that gets you to y.
to solve for this number, you divide y by x.
21 ÷ 3 = 7
this means to go from x to y, you multiply x by 7.
7x = y
we can use this to solve for x when y = 42
7x = 42
divide both sides by 7
x = 6
when y = 42, x = 6