Please help me!

To rent a midsize car for one day, Speedy charges a flat rate of $41 plus an additional fee of $0.10 per mile driven. Write an equation that relates the total cost of the car rental for one day, C(m), as a function of the number of miles driven, m.

I have finished this problem kinda and my teacher returned it to me and said you need to write a function and I'm kinda confused on what to do.

Is this maybe the function C(m) = 41(flat rate) + .10m(rate per mile)