First start out by finding his initial weight. If he’s gaining 5.5kg per month, and it’s after 11 months, 5.5 x 11 = 60.5kg gained during those 11 months.
To find his initial weight subtract 60.5 (which was gained) from 140 (the total). 140 - 60.5 = 79.5kg as the initial weight.
The initial weight represent the y-intercept, and 5.5kg is your rate (slope)
y = 5.5x + 79.5