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

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

So then when y=9, 9=x-21.

Solve for x.

x=9+21

x=30

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