Question 102752
A) Find the LCD, covert each rational expression into an equivalent expression with the LCD as the denominator. 
4/x-4 , 5/6-a 
lcd = (x-4)(6-a) because those denominators go not share a factor.
Converting you should divide the given denominator into the lcd
and multiply the numerator by the quotient:
For example: lcd/(x-4) = (6-a)
1st fraction becomes [4(6-a)/lcd]
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Then, lcd/(6-a)= (x-4)
2nd fraction becomes 5(x-4)/lcd
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B)If the given expressions were used as denominators of rational expressions, then what would be the LCD for each group of denominators? See example 3 
3/4b^2-0 2b/2b+3 -5/2b^2-3b
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Comment I think you have a typo in the denominator of the 1st fraction.
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Factor each expression as follows:

3/4b^2 , 2b/(2b+3), -5/[b(2b-3)]
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The lcd must contain each factor in its highest power:
lcd = 4b^2(2b+3)(2b-3)
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Cheers,
Stan H.