So, XZ= 7x+1 and PZ= 4x-1. To figure out XP we have to subtract those two.
(7x+1)-(4x-1)
3x-2
3(8)-2
24-2
22
XP=22
There 6 possible outcomes for a roll of a die. So if there are two dice, then it's 
-2x + (1/2)y = 18
(1/2)y = 2x + 18
y/2 = 2x + 18
y = 2*(2x + 18)
y = 2*2x + 2*18
y = 4x + 36
The x intercept is the value of x, when the y value is 0.
0 = 4x + 36
0 - 4x = 36
-4x = 36
x = 36/-4
x = -9
The x intercept is -9
I hope this helps.
This photo will show you how to do it and the answer so your welcome