Verified answer

3(a-7)-3(2a-4)=4(a+3)

3a-21-6a+12=4a+12

-3a-9=4a+12

-3a-4a=12+9

-7a=21

a=-3

Presumably you have to find the value of a - ?

3(a - 7) -3(2a - 4) = 4(a + 3) ......expand (i.e., distribute)

3a - 21 - 6a + 12 = 4a + 12.......collect terms

3a - 6a -4a = 12 + 21 - 12

-7a = 21

a = -3

3(a-7)-3(2a-4)=4(a+3)

3a - 21 - 6a + 12 = 4a + 12

3a - 6a - 4a = 12 -12 + 21

- 7a = 21

a = -3