First, write the equations in slope intercept form:
y=mx+b
where:
m = slope
b= y-intercept
-3x-9y=18
4x+3y= 12
-9y=18+3x
y= (3x+18)/-9
y= -1/3x-2
4x+3y=12
3y= 12-4x
y= (-4x+12)/3
y= -4/3+4
system:
y= 1/3x-2
y= -4/3+4
Graph both equations:
Where the 2 lines cross each other is the solution:
x=6
y=-4