To me this question means: find the equation of the line passing through (5,1) with a gradient, m=2.
The equation is y-1=2(x-5) because setting x=5 and y=1 (the point (5,1)) makes 0=0, which is true, that is, it is an equation (LHS=RHS).
So, y=2x-10+1=2x-9 is the equation in simplest form.