Firstly, the graph has a negative gradient (it plots down to the right side rather than up to the right side). This means that in terms of y, x must be negative (this cuts out two of the four options!) Next, you can just take a coordinate from the line, and match it to one of the equations! So take for example (1,-3). When x=1, y=-3(1). So, the answer is the third equation down!