My attempt: due to symmetry along x-axis, net E is only along x^hat.
dQ = λ dl = λ (R dθ)
for each dl, the x component of distance from dl to the origin is Rcos(θ)
Hence, E_x = \int_{-θ_0}^{θ_0} k(λ R dθ)/(Rcos(θ)^2 = kλ/R \int_{-θ_0}^{θ_0} sec(θ)^2 dθ = 2kλ/R tan(θ_0) along negative x^hat...