Problem with differential equation

Hello people I have a problem with this exercise.

I need to show that a motion of a pendulum can be found by solving the differential equation

with x=0 at t=0

The problem arises when I try to rearrange the equation and calculate the integral of which brings me nowhere. Anyone has any idea on how to solve this differential equation?

Also, I can't do the next part of the question, which is using Eulers method with t=0.1 to estimate x(0.3) if I don't find first a solution to the equation.

Any help would be greatly appreciated