So, if you plotted the graph of y=2x-1, gradient will be 2, y-intercept will be -1
It seems that the graph you plotted in the exercise could be written as y = 3x+c (you didn't tell us the value of c - it might be zero (?) , in which case the straight line passes through the origin ...)
I drew a graph and had to measure the gradient of the line, my answer was 3. The next question asked why did my answer of 3 probably not come as any surprise? Does anyone know what the any could possibly be? The graph has a right angled triangle if that has anything to do with it?