What is the solution of the system ?
2x+3y=4
x-y=-3

Change the equation, x - y = -3, to isolate x or y and plug that new equation into the equation 2x + 3y = 4

I changed it to: x = y - 3
so 
2(y - 3) + 3y = 4
2y - 6 + 3y = 4
5y - 6 = 4
5y = 4 + 6
5y = 10
y = 10/5
y = 2

then you plug that 2 into the equation x = y - 3 to get the value of x

x = (2) - 3
x = -1

you can check this by plugging the x and y values into the equation 2x + 3y = 4

2(-1) + 3(2) = 4
-2 + 6 = 4
4 = 4

hope this helps :)