Recall that slope = rise / run. If I were to choose the 1st and the 3rd points from which to calculate the slope, I'd get:
1 - (-2)
m = --------------- = 3/4
4
Let's use the slope-intercept form: y = mx + b
Subst. 3/4 for m, 2 for x and 1 for y: 1 = (3/4)(2) + b, or
1 = 3/2 + b, or 2/2 = 3/2 + b, or b = -1/2. Then the desired eq'n is
y = (3/4)x - 1/2