For question 21 ... I need the answer and explain how I got the answer !!!

Answer:
27=3c-3(6-2c)
Open the brackets (multiply 6 and -2c by -3)
27=3c-18+6c
Add 3c and 6c
27=9c-18
Add 18 to both sides
45=9c
Divide by 9
c=5 (45/9)
now to check this just substitute c (5) into the equation
27=(3*5)-18+(6*5)
27=15-18+30
27=30-3
27=27