To find the equation of the line that passes through the given points, you first must find the slope using the slope intercept formula (y2-y1)/(x2-x1) which is used by plugging your coordinate values into the formula. This would look like this:
(5-1)/(3-1)
When simplified, you should get 4/2 or (when simplified) 2 as your slope.
Now that you have this, you can plug in one of your coordinates and your slope into y=mx+b to solve for b. Since I am using the coordinate (1,1), the equation would look like this: 1=2(1)+b
When solved for b, you should get b=-1
Finally, now that you have the b and m (slope) values, you can write your equation as y=2x-1