Hi there!
First of all, let the unknown number be z.
Now, subtract 10:
z-10
This expression is equal to twice the same number:
z-10=2z
Solve this equation:
z-2z=10
-z=10
Divide both sides by -1 in order to get rid of the minus sign:
z=-10
Hope everything is clear.
Let me know if you have any questions!
#KeepLearning
:-)