The issue with using the side-splitter to solve for x is that the side-splitter theorem is used to find lengths of the sides being split by parallel lines. In this case, the x is on a line that is parallel and thus could not be solved for using this theorem.
If we created a triangle where the tops of the non-parallel lines connected, we would be able to compare the following and solve:

However, none of this would help us solve for x using this theorem.
It appears that you have chosen the correct answer to me.
Answer:
the value of x is 5 :) lalal
Answer:
-(5x-21)/6 so d
Step-by-step explanation:
to do inverse functions you want to replace the x in the function with y so x=(21-6y)/5, then solve for y and you will get the answer I got and its d
Answer:
im sorry i cant see that
Step-by-step explanation: