
; you first need to find out how much he'll gain in one month. If he went from 90 kg to 138, you should subtract

. This means he gains 48 in 8 months. Next, you should divide

, so he gains 6 kg/month. Since he starts out with 90 kg, 90 is the y-intercept.

will equal the time in months, and the slope is

-- or
.