Find (LCD) of 56 and 78 .
I think you really want the HCF Highest common factor
I'll use the web2 calc to help
factor(56) = 2^3*7 =2 *2*2*7
factor(78) = (2*3)*13 = 2 *3*13
The only common factor other than one is 2 so 2 is the highest one.
OR, if you meant "LCM", Least Common Multiple, then:
56 = 2^3 * 7
78 = 2 * 3 * 13
=2^3 x 7 x 3 x 13 =2,184.
