so y=mx+b
m=slope
b=y intercept, or the value of y when x=0
when x=0 is when time=0
that is initila value of 80
b=80
the slope is the increase per time unit
that is 15
so m=15
y=15x+80 is da equation where x is number of months elapsed
at the end of the year is 12 months elapsed
x=12
y=15(12)+80
y=180+80
y=260
after 1 year, he has $260