10=−9−x10=-9-x
1
Add 99 to both sides.
10+9=−x10+9=-x

2
Simplify  10+910+9  to  1919.
19=−x19=-x

3
Multiply both sides by −1-1.
−19=x-19=x

4
Switch sides.
x=−19x=-19

Done

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