How do you solve this step by step ?

Remembering that "slope" signifies "rise" over "run," read off the rise and run from the illustration.  If we start at x=-4 and move to the right 4 units, x becomes 0.  The "run" is thus 4.  At the same time, y increases from 0 to 2, and so the "rise" is 2.  

The slope is rise/run = 2/4 = 1/2.

The y-intercept can also be read off the graph:  Find the y-value on the x-axis where the graph (the sloping line) crosses the y-axis.  It's 2.

Thus, the equation of this line is  y = mx + b, or y = (1/2)x + 2.