Answer:
f(n) = 10 + 3n
Explanation:
Each of the expressions has 10 + a certain number of 3's
For example for n = 3 we have
f(3) = 10 + 3 threes = 10 + 3 x 3 = 10 + 9 = 19
For n = 4, we have
f(4) = 10 + 4 threes = 10 + 4 x 3 = 10 + 12 = 22
The others follow the same pattern
So the explicit equation is
f(n) = 10 + 3n
Testing on n = 6 we get
f(6) = 10 + 6n = 10 + 6 x 3 = 10 + 18 = 28