There are about 30 days in each month. with that in mind, we can say Species A grows about 3cm/30days and Species B grows 8cm/30days. As a function of days "d" we can say Species A grows at a rate of (3/30)d and Species B at (8/30)d.
*Notice how when d = 30, it reflects one month of growth for each.
And since each plant had some height already, we can write two equations like this:
Species A; H(d) = (1/10)d + 25
Species B; H(d) = (8/30)d + 10
*Just in case you didn't see it, (3/30) can be reduced to (1/10); which is what I did above.
