Question

the difference of a number and twice another number is 12. The sum of the two numbers is -27. What are the two numbers?

Answer 1

Let the numbers be x and x.  Then x-2y=12.  Also, x+y= -27, so that x = -27 - y.

Subbing the 2nd equation into the first gives us   -27 - y - 2y = 12, or

-39=3y, or y = -13 (answer).  Now find x from x = -27 - y.