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.

y = mx + c m is the gradient and c is the y-interceptUse values for A, x=5 y=9 and m=3so 9 = (3 x 5) + c 9 = 15 + c c = -6Then use coordinates for B, x=d y=15 and m=3so 15 = (3 x d) -6 15 = 3d - 6 21 = 3d 7 = d