Given that you say the question doesn't state the duration of work, I suspect it is asking for an equation to describe E in any situation.
Therefore, let's take x to be the number of hours he is working for.
We know he earns a base $250, so E is going to be 250+something.
That something we know is a multiple of the number of hours he is working for. If it is $40 per hour, it is going to be 40x as if x is 1 that gives 40, if it is 2 it gives 80 and so on.
If we substitute this all in we get:
E = 250 + 40x
Where x is the number of hours he works for.
