Answer:
Step-by-step explanation:
Apply the Pythagorean Theorrem. Find the sum of the squares of the two shortest sides and determine whether this sum equals the square of the longest side:
#19: 5^2 + 12^2 = ? = 13^2
25 + 144 = ? = 169 This is true, so you do have a right triange in #19.
#21: 2^2 + 4^2 = ? = 7^2, or 4 + 16 = ? = 49 This is false. Not a right
triangle.
Apply this same approach (Pythagorean Theorem) to the remaining problems.

if you have already covered slopes, you could also get it that way, in fact is simpler that way.
Answer:
the triangles are not similar.