Question

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

Answer 1

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

Answer 2

a-4b=4b
a=8b
Plug it in:
24b+4b=21
28b=21
b=21/28
a=8b=6.