Suppose y varies directly as x and y= 21 when x= 3 find x when y=42
what is an equation for it

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