Slope is the change in y over the change in x, or
y2-y1/x2-x1
It doesn't matter which x and y coordinates we use as x1 and x2 as long as we are consistent with both. Since the x component in the slope = 3, we can see that they used 7-4, so for the y component we will use w - (-6)
w-(-6) = 4 (we set it to 4 because of the slope)
w+6 = 4
w = 4-6
w = -2
The missing point is -2