The money we would have at any time can be modeled as
M = 27k + 141
Why?
you started with $141, so that is the base amount,
every month you add 27 dollars,
in one month you add 27 dollars,
in two months you 27 again making 54 dollars,
so , in x months, you have added 27x dollars to the 141 dollars,
thus our equation is
M = 27k + 141