The 2-point form of the equation for a line can be used.
... y = (y₂-y₁)/(x₂-x₁)·(x -x₁) +y₁
Filling in the given information, you have
... y = (4-1)/(4-3)·(x-3) +1 . . . . an equation for the line
... y = 3x -8 . . . . . . . . . . . . . . simplified to slope-intercept form
... 3x -y = 8 . . . . . . . . . . . . . .. rearranged to standard form
