Question

If y=-7 when x=-21, what is the value of x when y=9

Answer 1

y=-7=x+14 if x - -21

So then when y=9, 9=x-21.
Solve for x.
x=9+21
x=30

Answer 2

You'd be well advised to identify the type of problem you're working with.  Is this problem one of direct proportion (as I think it is)?  If so, say so.

If y = -7 when x = -21, then y  = (1/3)x, and (1/3) is the constant of proportion.

A similar truth is that x = 3(y), as -21 = 3(-7)

So, if y=9, x = 3(9) = 27