March, April, and May correspond to 3,4, and 5th months. Each month the amount of money he saves increases by $40, so this is a constant rate of change, or constant velocity, which means that this is a linear equation. Let y=the amount of money he has and x=the month number and model it with the line of form y=mx+b, where m=slope, or rate, and b=y-intercept...
y=40x+b, we know that he has $40 on the third month (February) so:
40=40(3)+b
40=120+b
-80=b so the equation is:
y=40x-80 (note that the domain is x=[3, +oo) only for the function to have meaining)
Now you want to know when he has $320...
320=40x-80
400=40x
10=x
So he will have $320 on the 10th month, which is October.