#### Answer

LCD is 168.

#### Work Step by Step

To find the least common denominator, factor each number. For each unique factor, identify the maximum number of times it occurs in any one of the factorizations and include it that many times when computing the LCD.
$3=\underline3$
$21=3\times\underline7$
$56=\underline2\times\underline2\times\underline2\times7$
$LCD=3\times7\times2\times2\times2=21\times4\times2=84\times2=168$