It appears the graph goes through (0,3) and (5,6)
If so, then you have the correct slope and y intercept.
The formula is
which is the same as writing C = (3/5)n+3
Don't forget about the parenthesis around the 3/5.
We can verify this by plugging in n = 0 and you should get C = 3.
Also, if n = 5 then it leads to C = 6.