The 2-point form of the equation of a line is
.. y = (y2 -y1)/(x2 -x1)*(x -x1) +y1
For your points (x1, y1) = (0, 3) and (x2, y2) = (3, 5), this is
.. y = (5 -3)/(3 -0)*(x -0) +3
.. y = (2/3)x +3
Written as a function, it is
.. f(x) = (2/3)x +3
.. y = (y2 -y1)/(x2 -x1)*(x -x1) +y1
For your points (x1, y1) = (0, 3) and (x2, y2) = (3, 5), this is
.. y = (5 -3)/(3 -0)*(x -0) +3
.. y = (2/3)x +3
Written as a function, it is
.. f(x) = (2/3)x +3