The answer will depend on what the problem is asking you to solve for...
For example, if it is asking you to solve for the x-intercept then you would set y equal to 0 and solve for x:
y = -x - 3
0 = -x - 3 add x to both sides
0 + x = + x -x - 3
x = -3
Thus, the x-intercept is (-3, 0)