It is wrong. Substitute the values with proper sign.
You forgot that you are subtracting negative numbers. Recall that x - (-y) = x + y.
Also, the formula for slope is (y_{2} - y_{1}) / (x_{2} - x_{1}). There is a big difference between this and y_{2} - (y_{1} / x_{2}) - x_{1}.
If you mean (3- (-2))/(1-(-2)) then you are not doing what you were told before. 3-(-2)= 3+ 2= 5. 1-(-2)= 1+ 2= 3. The slope is 5/3. No, the midpoint is NOT "(-0.5, 0.5)" nor does that point lie on the line. You were given points (-2 , -2) and B (3 , 1). The midpoint is ((-2+3)/2, (-2+1)/2)= (1/2, -1/2) or (0.5, -0.5). I don't know whether you are confusing x and y, using an incorrect formula, or just doing the arithmetic wrong.
the point (-0.5,0.5 does lie on the line on my graph? Apparently in the middle. For this I used the formula (1/2(x_{1}+x_{2}) , (1/2(1_{1}+2_{2}).
So I have 3 points, A (-2 , -2), B (3 , 1) and C (6 , 2)
I have calculated slope of line AB = 3/5 with a midpoint at (0.5 , -0.5) and slope of line CB = -1 with a midpoint of (4.5 , -0.5).
All points satisfy a graph with axis scaled at 0 - 0.5 - 1 -1.5 Etc.