Addition and Subtraction of Rational Expression must have the same denominator.


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.





Since the denominator are the same, you can subtract the expressions.