You may want to review finding intercepts first.
Sometimes, you might have to find a point on a line for x = 3, say. If it were a 0 instead of a 3, then this would just be asking for the y-intercept, and you'd know what to do; just put 0 in for x and solve for y. In this case, it's the same idea; just put a 3 in for x instead of a 0.
For example, if you have to find a point on 2x + y = −1 with x = 3, you just put 3 in for x, getting 6 + y = −1. Solving for y gives y = −7, so your point is (3, −7).
If instead you wanted y = 3, you'd just put 3 in for y instead of for x. In this case, you'd get 2x + 3 = −1, whose solution is x = −2; so the point is (−2, 3).