A is the point with coordinates (5, 9) B is the point with coordinates (d, 15). The gradient of the line AB is 3. Work out the value of d.

gradient is calculated using the change in y coordinates divided by change in x coordinates. so, (15-9)/(d-5)=3 => 6/(d-5)= 3rearrange this equation to get d on one side (6/3)+5 = d therefore d= 7