Electric field on the axis of a uniformly charged ring at 1.15 cm

[ILoveCollege]

Homework Statement

A uniformly charged ring of radius 8.1 cm has a total charge of 118 micro Coulombs. The value of the Coulomb constant is 8.98755e9 N M^2/C^2. Find the magnitude of the electric field on the axis of the ring at 1.15 cm from the center of the ring. Answer in units of N/C.

Homework Equations

F= k Qq/ r^2
E= kq/r^2

The Attempt at a Solution

I tried subtracting 1.15 cm from 8.1 cm for "r" and plugged that "r" value in the F equation but that answer is wrong. By axis , do they mean horizontally (as in along the diameter) or vertically?

 

[ILoveCollege]

do i just do the pyth. theorem to solve for "r" ( square root of 8.1^2 + 1.15^2)?