They are trying to use the property of similar triangles.
For that reason, we first need to represent the 2 triangles such that they are similar (meaning, rotate the triangles in such a way, that the corresponding angles in both the trriangles match)
For that, we need to rotate the left triangle, to match the angle ‘x’ in both the triangles.
So the base changes.
thank you for sparing some time and helping but like after rotating how would we match x? and how to know when base changes in which all case?
thank you!
If we rotate the left triangle by 90 degrees clockwise, we will get left angles for both triangles as 90, bottom right angle for both the triangles as ‘x’, and the remaining angle also same for both the triangles.
In a way, we are just mapping that the corresponding angles in both the triangles should be the same.
This proves that they are similar, and then of course we can use the similarity property (of equal ratios of sides of the 2 triangles)