Since [tex]\Delta x=\dfrac{b-a}n[/tex], and you have a corresponding term in the given Riemann sum of [tex]\dfrac5n[/tex], you know the integral is being taken over an interval of length 5, so you can omit the second choice.
Next, [tex]f(a+i\Delta x)[/tex] corresponds to [tex]a=3[/tex] with [tex]\Delta x=\dfrac5n[/tex]. The fact that [tex]a=3[/tex] alone tells you that the interval of integration starts at 3, and since we know the interval has length 5, that leaves the first choice as the correct answer.
