The first step is to add 4x to 5x, which gives you 9x. This is because co-interior angles add up to 180, so 4x+5x=180. Co-interior angles can be spotted by their C shape.
To work out the value of x you would need to divide 180 by 9, which gives you 20°. This is because 9x is equal to 180. This means that x is equal to 180 divided by 9.