If 3a+4b = 4a-4b = 21 find the value of a

A-4b=4b

a=8b

Plug it in:

24b+4b=21

28b=21

b=21/28

a=8b=6.

Using the 2nd and 3rd you have:

4a-4b=21

4a=21+4b

a = (21+4b) / 4

now using 1st and 2nd

3 (21+4b) / 4+4b=4 (21+4b) / 4-4b

(63+12b+16b) / 4 = (84+16b-16b) / 4

63+28b=84

28b=21

b=21/28

b=3/4

... now using 4a-4b=21 we have

4a-4 (3/4) = 21

4a=3=21

4a=24

a=6