LCD=least common denominator, is in fact the least common multiple (LCM) of the denominators.
Lets first break down the denominators into factors:
4x^2=2*2*x*x
7x^2+14x=7*[x]*(x+2)
out of which I have enclosed in square bracket duplicated factors between the two denominators.
The least common denominator is there the product of the remaining factors:
2*2*x*x*7*(x+2)
=28x^2(x+2)