Answer :
If you meant to write 'a' instead of 's':
-7(5a-6k)
-7(5[8]-6[3])
-7(40-18)
-7(22)
-154
Brackets first! :)
-7(5a-6k)
-7(5[8]-6[3])
-7(40-18)
-7(22)
-154
Brackets first! :)