based on what I can see on the image, it would appear it's just asking for what each boy pays for at the beginning of their gym fees, and how much it goes up by.
you can see that the starting value (when the month(m) = 0)
so for Alfredo, you can see from the chart that the initial value is 20.
for Alex, when you plug 0 in for m, you get c=25+10(0)=25
➡ To figure out the rate of change, you just see how much it increases each month
for Alfredo, you subtract month 1 from month 2, which gives you 10
for Alex, you plug in 1 and 2 for m. then you subtract the result of 1 from 2
c=25+10(1)=35
c=25+10(2)=45
45-35=10
rate of change = 10
I hope that made sense