Combine the like terms to create an equivalent expression:
 {-k-(-8k)}−k−(−8k)
  {-k-(-8k)}−k−(−8k) 
Apply PEMDAS (Parentheses, Exponentiation, Multiplication & Division, Addition & Subtraction), and remember operations at the same level (e.g. + -) are performed left to right:
(showing lots of steps, most of which you can do in your head w/practice)
{-k-(-8k)}−k−(−8k)
= { 7k } -k - (-8k)
=  7k  - k - (-8k)
=  6k - (-8k)
=  6k + 8k 
= 