From the table given
We can see that
when x=3, f(x)=2.5
when x=6, f(x) =4
when x=10, f(x)=6
when x=15, f(x)=8.5
If two coordinates are selected, then we can find the relationship by using the equation below
If we select the points
x=6 and f(x)=4
x=10 and f(x)=6
Therefore, If we substitute these values
Answer=>
y=1/2x+1