Point M has coordinates (6, 3p−1) and point N has coordinates (1−p, 2). The gradient of the straight line joining M and N is −3 and it crosses the y-axis at (0, r). What is the value of r ?

i used y=mx + c, or tried to at least, but i did not get any solid answer

since the line crosses the y axis at point (0,r), y=-3x+c can be written as y=-3x+r
plug in the values of x and y for point M and N:
for N : 2 = -3(1-p) + r
2 = -3(1-p) + r
2 = -3 + 3p + r
p = (5-r)/3

for M replacing p with (5-r)/3:
(5-r)/3 - 1 = -3 * 6 + r
11 = r