You have the following equation:
x - 4y = -18
In order to graph the line of the equation, you first solve for x (you can also solve for y but in this case is easier for x):
x - 4y = -18
x = 4y + 18
By assigning three different values to y, you can obtain values for x, and then, you can obtain three points to graph.
x = 4(-6) + 18 = -24 + 18 = -6
x = 4(-4) + 18 = -16 + 18 = 2
x = 4(-3) + 18 = -12 + 18 = 6
Thus you have the following three points:
P1(-6,-6), P2(2,-4), P(6,-3)