One fourth of the opposite of the difference of five and a number is less than 20

Find two numbers if their sum is 91 and the ratio is 6: 7?

I’m still confused on how to work the percentages

Simplify (4x^2 - 8xy + 2y^2) - (9x^2 - 4xy - 7y^2) a. -5x^2 + 4xy + 9y^2 b. -5x^2 - 4xy + 9y^2 c. -5x^2 + 12xy + 4y^2 d. -5x^2 - 4xy - 5y^2