how do i find f(x)

[sharp moon]

look to the general formula for a right Riemann sum:

xi=a+i∆x

a=6

notice the pattern in your series

and delta X is 3/n

therefore your function is sin(x)

because with the bottom one your function is sin(6+x) so f(xi) is sin(6+a+i∆x) but a is 0 and ∆x is 3/n so sin(6+3/n)