Solve by the "addition-or-subtraction method".?

subtraction!
because the -5b minus another -5b cancels out the second negative and makes it a positive, so -5b+5b=0 then you have one to solve for at the time, then fill it in and solve for b

12a - 5b = 18 ...... (1)
15a - 5b = 30 ...... (2)

(1) - (2).
....12a - 5b = 18
–) 15a - 5b = 30
------------------------------
.. ..-3a ......= -12

-3a = -12
a = -12/(-3)
a = 4

12a - 5b = 18

Substitute 4 for a.
12(4) - 5b = 18
48 - 5b = 18
5b = 48 - 18
b = 30/5
b = 6

∴ a = 4, b = 6

- 12 a + 5 b = - 18
15 a - 5 b = 30 -------ADD

3a = 12
a = 4

- 48 + 5 b = - 18
5b = 30
b = 6

a = 4 , b = 6

y u haven pple do your work?

sub eq 1 from 2 ie 15a-5b-(12a-5b) =12
3a=12 a=4 therefore b =6

12a - 5b = 18 ... eq.1
15a - 5b = 30 ... eq.2

eq.1 - eq.2,
(12a - 5b) - (15a - 5b) = 18 - 30
12a - 5b - 15a + 5b = -12
-3a = -12
∴ a= 4

substitute a=4 into eq.1,
12(4) - 5b = 18
48 - 5b = 18
-5b = -30
∴ b= 6

∴ a= 4 and b= 6

Hope that helped! :)