-(a-7)=-2(4-a)

The 'stock' answer is : 'a' is the first letter of the alphabet !

In this instance : -(a-7) =-2(4-a)   Remove the parentheses to get:

-a + 7 = -8 +2a       Now add 'a' to both side to get

7 = -8 + 3a              Now add '8' to both side to get

15 = 3a                   Now divide both sides by '3' to get

5 = a    

~jc