Weare given two examples of statements that are not true, so we can pick a counterexample to show they are FALSE:
In the case:
"All snakes are poisonous."
We can give the boa or the anaconda as counterexamples. since they don't have poisonous glands.
In the case:
If two angles are complementary, then they are congruent, we can pick as counterexample the angles of measure 30 degrees and 60 degrees which added give a 90 degree angles (and therefore are complementary), but are not congruent since they don't have the same measure.