[tex]W(t) = 6t+90[/tex]; 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 [tex]138-90=48[/tex]. This means he gains 48 in 8 months. Next, you should divide [tex]48/8=6[/tex], so he gains 6 kg/month. Since he starts out with 90 kg, 90 is the y-intercept. [tex]t[/tex] will equal the time in months, and the slope is [tex] \frac{6}{1} [/tex]-- or [tex]W(t) = 6t+90[/tex].
