2a = b
a -3b
=12
To solve, set one of the equations equal to zero. The
first equation is actually already set to 0.
b =
2a
Now you can plug this equation into the other one, by
substituting the b in the equation with this.
a -3b =
12
a – 3(2a) = 12
Now all you
have to do is distribute the 3 (multiply the number outside the parenthesis, 3 in this
case, by everything inside).
a – 6a =
12
Combine like terms. Since we have 1 positive a and 6
negative ones, we subtract the two numbers without the
signs
6-1 = 5
And take the
sign of the larger number. Since 6 is large and 6 is negative, we are left
with
-5a = 12
Now solve for a,
which in this case means dividing both sides by -5
a =
-12/5
Simplify
a =
-2.4
Now that you have a, it’s easy to find
b
b = 2a
b =
2(-2.4)
b= -4.8
Now you check
your work by plugging your new a and b into either equation to see if it’s
true.
-4.8 = 2(-2.4)
-4.8 =
-4.8
And just for laughs let’s try the other
equation
a -3b
=12
-2.4 – 3(-4.8) =
12
Distribute the 3 again. Remember that when two negative
numbers are multiplied they become positive.
-2.4 +14.4 =
12
12 = 12
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