Draw the other line and write down the value of x and y at the point of intersection. Those values are the answer because when you are solving the set of simultaneous equations, you are essentially solving the two lines together.
A simple example would be:
Given x = 2 and y = 1 - 2x, solve for y.
In this case, we are given the value of x.
Without the graph, you would solve manually aka replace the value of x in y =1 - 2x with x = 2. You will then arrive with the value of y, y = - 3.
The other method would be the graphical method. x = 2 is a vertical line while y = 1 - 2x is a linear line. Plot both lines on a graph, and you will realise that the point of intersection is (2, -3). The y-coord is the value that you would have attained if you solved the two equations manually.
The only difference between the above example and your question is that you are presented with 2 linear equations.
Hope this helps!