You start by finding two points on the line. In this case, (-4,1) and (-2,2) will do.
To get from (-4,1) to (-2,2), you need to go “up 1, right 2” which gives you a slope of m = 1/2
Next you need the b-value, which comes from the y-intercept of (0,3). The b-value is 3.
Putting the slope and b-value into y=mx+b, you have y = 1/2 x + 3.