formula: y=mx+b
1) looking at the line, we see the line crosses the y-axis at (0,3). this is our y-intercept, which is b in our equation.
y=mx+3
2) since the slope is positive (going up on the right), look to the right of the y-intercept and find the next point on the graph the line goes through. the next point is (2,4). all you have to do is count how many units up and across you went to get from one point to the next. so we did 1 up and 2 to the right. so, using rise/run, we have 1/2. this is b in our formula.
y=1/2x=3