You have to combine like terms.
Answer: -2a+7
Answer with Step-by-step explanation:
a.

Taking both sides log

Using identity:

Using identity:


b.
We know that

Using identity

c.

Substitute the values then we get

By using 
Hence, 
Multiply first, then add. 12 times 7 = 84. Then add 8. 84 + 8 = 92.