You can in this case simple subtract one equation from the other to eliminate the y variable...
(3x+y=4)-(2x+y=5)
3x-2x+y-y=4-5
x=-1, now you can substitute this value of x into either of the original equation to find the value of y...
3x+y=4 becomes
3(-1)+y=4
-3+y=4 add 3 to both sides
y=7
So the solution to the system of equations, where graphically the two lines intersect, is the point:
(-1, 7)
