Question 149801
This is poorly worded question.

First question, is this really correct f(x) =  5x +  ??? That seems like it is missing a term after the plus sign.

Second question, f(x) = -38  is, in fact, a second function. It defines the vertical line that runs through (-38,0). 

What I think they meant to ask was f(x) = 5x + (somethingGoesHere) ; f(X) = -38
In this case, the X in the second expression is not a variable, but a specific instance where x=X.

So then all you need to do is:
f(x) = 5x + (somethingGoesHere)
f(X) = 5X + (somethingGoesHere)
-38 = 5X + (somethingGoesHere)
(-38 -(somethingGoesHere))/5 = X

I hope this helps