To write an equation, it is enough to know the rate of change (slope) and the initial value (y-intercept).
The equation of a line of slope m and y-intercept b is:
For Ms. Mistovich, the initial value is 4 and the rate of change is 2.
For Ms. Nelson, the initial value is 1 and the rate of change is 3.
Therefore, the equations that model this situation, are: