7th grade math!brainliest answer!

y varies directly with x, and y = 7 when x = 2. what is the value of x when y = 21? enter your answer in the box.

If y varies directly with x, the formula of the equation is y=ax, where a is a constant.
y=ax
(21)=a(7)
a=3
y=ax
y=(3)(-1.66)
y=-4.98, x=-1.66

If y varies inversely with x, the formula of the equation is y=a/x, where a is a constant
y=a/x
9=a/2.5
a=22.5
y=22.5/x
y=22.5/-0.6
y=-37.5

I hope this helped! =)
I hope you didn't mind but I got this from a website called: answers.yahoo.com but I got your answer.

Y = kx is the direct variation equation
Fill in the x and y given to solve for k. Then write a new equation with k to solve for future equations.
7 = 2 * k
7/2 = k

y = 7/2 x

21 = (7/2) x
(2/7)(21) = x
6 = x

Ifound this on google “that means that the graph of a function with direct variation has these features: it is a straight line: because the slope is the ratio y/x = k. it passes through the origin (0,0): because y = kx = k (0) = 0.” i hope this you for what your looking for