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.