Case 1: $(a.b)(c.d)$ 

Case 2: $(e+f)(g+h)$

In both cases if you a value for the for each letter just calculate each bracket and multiply by the value of the second bracket. 

If no values are given Case 1 is still relatively simple as multiplication is commutative and the only operation is multiplication is the only operation so $(a.b)(c.d)= abcd.$

If no values are given in Case 2 it is more complicated. Whilst addition is also commutative it is not the only operation. Obviously I have been taught 
$(e+f)(g+h)= eg+eh+fg+fh.$ How was this result arrived at?