take a derivative of y with respect to x. This represents the slope of a line tangent to the graph at a given point. It gives you the point (1,3) so plug in x = 1 in dy/dx this gives you the slope of the tangent line. You know a point that the line passes through, and you know its slope, so you can get the equation of the line with y = mx + b, and of course whatever b turns out to be is your answer.
(i got b = 4)