First you need to reduce each square root. In each case, find the largest perfect square factor under the radical.
in the case of sqrt(20), that boils down to 4*5, so sqrt(20)=sqrt(4)*sqrt(5), or 2sqrt(5).
sqrt(80) = sqrt(16)*sqrt(5) = 4sqrt(5).
Final answer: subtract 4 sqrt(5) from 2 sqrt(5). Can you do this?