Recall that the slope or gradient of a line segment through two points is the ratio of the change in the y coordinate to the change in the x coordinate.
If the gradient is 1, then
1 = (-3 - 4) / (a - 3a)
Solve for a :
1 = -7 / (-2a)
1 = 7/(2a)
2a = 7
a = 7/2
