Okay, so, first, we plug in each number for each variable, for example, in your equation:
b - 2a - c
_________
a = -3
b = 9
c = -6
_________
Okay, so now, we plug in each number for each variable:
9 - 2( -3 ) - ( -6 )
Now, we do PEMDAS:
Parentheses
Exponents
Multiplication
Division
Addition
Subtraction
-2 times -3 equals 6, because, two negatives make a positive:
9 + 6 - ( -6)
think of the - before the parentheses, as a negative one, and, again, two negatives make a positive, so, -1 times -6 equals 6
9 + 6 + 6
Now, we solve by addition, because they are all like terms:
9 + 6 + 6
15 + 6
________
21
The answer is 21.
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I hope this helps!
