Saying that y varies directly with x is written mathematically as

y = k.x

where k is some constant.

Using the data values you are given, we can write   14 = k.(-4)

and re-arranging to make k the subject of the formula, we have

determined that k has the value -3.5

So the formula in this exercise is now knwn to be y = (-3.5)x

So we have now determined that the formula in this exercise is 

that is a great answer.  thanks anon.