I assume that you're trying to put this in slope intercept form.
To do so, you must isolate y; therefore, you must subtract 2x to get it on the other side, where -4 is. Once that's done, you'll have -y = -2x - 4
Now, y cannot be negative, so we must divide it by -1 in order to change y to a positive. Afterwards, we have y = 2x - 4
y = 2x - 4 is your answer, that is, if trying to put it in slope intercept form.