To find for the number of fireflies caught in a minute, we simply look at the equation. The equation is linear and takes the form y = slope * x + b
where slope is also equivalent to the fireflies caught each minute.
So the equation of you and your friend is: (where y is the total fireflies caught)
you:
y = 3 m + 13
your friend:
y = 4 m + 6
So the fireflies caught each minute is:
you: 3
your friend: 4
So a total of 7 fireflies each minute combined