Here ... I'll give you all the help you need to answer this question
on your own:
-- The three angles inside any triangle always add up to exactly 180 degrees.
-- Simply examine each group of numbers in the question.
Any set that adds up to 180 can be the three angles of a triangle.
Any set that doesn't, can't.
Go to it !
When you write that equation, you get 3x^2+6=81.
Solving for x:
Subtract by 6 on both sides of the equation to get 3x^2=75.
Then, divide by 3 on both sides of the equation to get x^2=25.
Finally, do the square root on both sides of the equation to get x=5.
The number is 5.
D. You have 4.75 flat and 1.5 every time you are driving another mile.