The straight line joining the points A(3,-5) and B(6,k) has a gradient of 4.
Gradient is the slope
So the slope of the line joining the points A(3,-5) and B(6,k) is 4
Slope of line joining two points =

A(3,-5) and B(6,k) are (x1,y1) and (x2,y2)
slope =

slope =

We know slope =4

Cross multiply and solve for k
k + 5 = 12
k = 7
The value of k = 7