$${E = \int dE_x = \int_{-60^{\circ}}^{60^{\circ}} \frac{1}{4 \pi \varepsilon_0} \frac{\lambda}{r^2} \cos \theta r d\theta \\ = \frac{\lambda}{4 \pi \varepsilon_0 r} \int_{-60^{\circ}}^{60^{\circ}} \cos \theta d\theta = \frac{\lambda}{4 \pi \varepsilon_0 r} [\sin \theta]_{-60^{\circ}}^{60^{\circ}} \\ = \frac{\lambda}{4 \pi \varepsilon_0 r} [\sin 60^{\circ} - \sin 60^{\circ}] \\ = \frac{1,73\lambda}{4 \pi \varepsilon_0 r}}$$