Question

Classify the triangle based on the side lengths 4.2, 6.4 and 7.6

acute
right
obtuse
no triangle can be formed with those sides. 

Answer 1

Right angle
4.2 squared + 6.4 squared = 58.6
Square root of 58.6 = 7.6 so you know the longest side is right
And since you used Pythagoras theorem you know it must be a right angle