I believe you are referring to exponents that have a multiple of three. If so any number * three is a valid exponent in your case.
The equation is b-a= 21;
2b-5a = 18
a=8 and b= 29
therefore 29-8= 21
2 (29) -5 (8) = 18
58-40=18
y = - 5
substitute the value of x = 4 into the equation and solve for y
- 12 + 9y = - 57 ( add 12 to both sides )
9y = - 45 ( divide both sides by 9 )
y =
= - 5
The correct answer is:
[D]: "

" .
__________________________________________________Explanation:__________________________________________________
;
Simplify the "denominator" ; → (b + 2b = 1b + 2b = 3b) ;
and rewrite:

;
Cancel out the "b" in the "numerator" to a "1" ;
and cancel the "3b" in the "denominator to a "3" ;
{since: "b ÷ b = 1 "; and since: "3b ÷ b = 3" } ;
and rewrite as:
____________________________________________________ 
;
____________________________________________________ → which is:
Answer choice: [D]: "
" .
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