Answer:
f(6) = 16
Explanation:
In the next steps we use this formula : f(n)=f(n-1)+2 
Since f(1)=6 then f(2)=f(2-1)+2 = f(1)+2 = 6+2 = 8
 f(2)=8 then f(3)=f(3-1)+2 = f(2)+2 = 8+2 = 10
 f(3)=10 then f(4)=f(4-1)+2 = f(3)+2 = 10+2 = 12
 f(4)=12 then f(5)=f(5-1)+2 = f(4)+2 = 12+2 = 14
 f(5)=14 then f(6)=f(5-1)+2 = f(4)+2 = 14+2 = 16.