I hope that 2-1 means 21.
DE + EF = DF
4x + 10 + 21 = 9x - 15
4x + 31 = 9x - 15
31 = 9x - 4x - 15
31 = 5x - 15
46 = 5x
x = 9 1/5 = 9.2
You want 9x - 15
9*9.2 - 15
82.8 - 15
67.8 <<<<< answer.
If I have given the wrong meaning to 2-1 (which could mean 1) leave another note and I will correct my work.
Answer:

Step-by-step explanation:
Using the section formula
=
=
=
= 
The example of a right angle is that it's exactly 90°, the acute angle is less than 90°, and the obtuse angle is more than 90° but less than 180°.
Answer:
(6,-3) or x=6 y= -3
Step-by-step explanation:
Assuming you mean 2x-8y=7, we can start by multiplying the first equation by -2 and adding it to the second (to cross out the x), resulting in -18y=-5. After that, we can divide both sides by -18, resulting in y=5/18. Next, plugging that into 4x-2y=9, we have 4x-10/18=9. Adding 10/18 to both sides, we get 4x=9+10/18. After that, we can divide both sides by 4, resulting in x=(9+10/18)/4=