You can put this solution on YOUR website!
There's no single solution, so you can't solve it.
The graph is a straight line, and every point on it is a "solution" in the sense that it fits the equation.
3x + 3y = 9
Find y in terms of x:
x + y = 3
y = -x + 3
Pick 2 values for x and find the corresponding y.
Plot the 2 points.
Draw a straight line thru them.