Question

if y=-2 when x=1 find x when y=4

Answer 1

In general, y = <<stuff happens to x>>

The problem is there are an infinite number of answers for what x =

Here's the simple answer:

If:

y = -2x

Then:

4 = -2x

(divide by -2 on both sides)

-2 = x

 

Here's a more complicated possibility:

y = x^2 - x -2

y = 1^2 -1 -2

y = -2

4 = x^2 -x -2

0 = x^2 -x -6

0 = (x - 3) (x + 2)

x = -2 or +3

 

You can continue making more and more complicated functions- y = <<more and more complicated stuff happening to x>>

The answer your teacher is probably looking for is x = -2, but the most technically correct (the best kind of correct) answer is probably x = any real number.