Question

If X + 2Y =18 , how am I supposed to find the slope if I don't know how to find the x intercept?

Answer 1

X+2y=18
-x      -x
2y=-x+18
/2   /2  /2

y=mx+b
y=-1/2x+9

the slope is -1/2

Answer 2

You solve for y. you dont need the x intercept 
x+2y=18
2y=-x+18
divide everything by two
y=-1/2x+9
the slope would be -1/2 
since in the equation y=mx+b 
m represents the slope which in this case would be -1/2