Answer:
17) Congruent because two sides are congruent and so is the 90 degree vertical angle.
18) Congruent because they have a congruent leg and congruent angle, and they share a side which is congruent to itself. (SAS)
19) Congruent because all three sides are congruent (SSS)
20) congruent, all three sides are the same (SSS)