These problems are based on the Pythagorean Theorem. The Pythagorean Theorem is a^2 + b^2 = c^2. If the two sides are equal, the triangle is a right triangle. If c^2 is less, the triangle is acute. If c^2 is more, the triangle is obtuse.
Obtuse: c^2 > a^2 + b^2
Acute: c^2 < a^2 + b^2
Right: c^2 = a^2 + b^2
#7 --- Obtuse
21^2 ___ 8^2 + 15^2
441 ___ 64 + 225
441 > 289
#8 --- Right
20^2 ___ 12^2 + 16^2
400 ___ 144 + 256
400 = 400
#9 --- Acute
6^2 ___ 4^2 + 5^2
36 ___ 16 + 25
36 < 41
Hope this helps!