Pay close attention, here
The triangle on the left is a 30-60-90 right triangle
The sde opposite the 30° angle = 1
The side opposite the right angle [ the dashed line] = twice this = 2
And the triangle on the right is also a 30-60-90 right triangle
The angle at the top right vertex is the 30° angle in this triangle
And the side opposite this = the dashed line = 2
And "x" is opposite side to the right angle in this triangle so it's twice as long as the dashed line = 4