To change 2x + y = -1 from standard form to slope-intercept form, you must take away x and its coefficient from both sides to get it on the other side.
If you had a positive x, then you'd subtract, and if you had a negative x, then you'd add on both sides.
The last step would be to divide by the coefficient of y on both sides, but y has no coefficient in this problem so we don't need to worry about that right now.
Steps to Solving This Problem:
2x + y = -1
-2x -2x
y = -2x - 1
That's all there is to it! Hope this helped.