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

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

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