f(x) = 2x - 1
To obtain a function we require values of x for x = 3, 4, 5 , 6, .....
The output appears to increase in steps of 2
3 → 5
4 → 7
5 → 9
6 → 11
Looking at the consecutive values of y we should see that these are twice the corresponding value of x subtract 1
f(x) = 2x - 1 ← is a possible function
checking the values for x in the input
f(3) =(2 × 3 ) - 1 = 5
f(4) = (2 × 4 ) - 1 = 7
f(6) = (2 × 6 ) - 1 = 11
f(11) = (2 × 11 ) - 1 = 21
