Question 68798
<pre>
Addition and Subtraction of Rational Expression must have the same denominator.
{{{3/2b  -  5/3b^3}}}

Since 2b and 3b^3 are not the same, we are going to find the LCD.

Let us get the prime factor for:
2b   = 2*  b
3b^3 =   3*b*b*b
LCD  = 2*3*b*b*b = 6b^3

Rewrite an equivalent expressions using the LCD.

{{{(3*3b^2)/6b^3  -  (5*2)/6b^3}}}

{{{9b^2/6b^3 - 10/6b^3}}}

Since the denominator are the same, you can subtract the expressions.
 
{{{(9b^2 - 10)/6b^3}}}</pre>