The slope is 7, because from 0 to 3, the increase is 21, from 3 to 5 it is 14 and from 5 to 7 it is also 14.
The starting value is 13, this is given from x=0.
So the equation is y = 7x + 13, but that's not in the list.
We'll have to manipulate it until we find the formula that's equivalent. If we move the 13 to the left and the y to the right (the signs swap as you know), we get:
-13 = 7x - y
Swap the left and the right side and you get
7x - y = -13, that's answer D!