SOLUTION: How do you solve by the addition-or-subtraction method. 12a-5b=18 15a-5b=30

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Question 173665: How do you solve by the addition-or-subtraction method.
12a-5b=18
15a-5b=30
Answer by jojo14344(1513) About Me  (Show Source):
You can put this solution on YOUR website!

12a-5b=18---->12a-5b-18, EQN 1
15a-5b=30---->15a-5b-30=0, EQN 2
Subtract EQN 1 to EQN 2:
where----->system%2812a-5b-18=0%2C-15a%2B5b%2B30=0%2Chighlight%28-3a%2B12=0%29%29 --> see EQN 2, change "signs" when subtacting right?
Continuing the "highlighted" eqn:
-3a=-12----->cross%28-3%29a%2Fcross%28-3%29=cross%28-12%294%2Fcross%28-3%29
highlight%28a=4%29
Go back EQN 1:
12%2A4-5b=18
48-18=5b --->30=5b--->cross%2830%296%2Fcross%285%29=cross%285%29b%2Fcross%285%29
highlight%28b=6%29
Check go back EQN 2:
15%2A4-5%2A6-30=0
60-30-30=0
0=0
Thank you,
Jojo