Polynomials Simplify each expression

Simplify the expression: (2x² + 3x - 5) + (7x² - 9)

Simplify the expression: (r² + 2t) + (r² + 6rt - 9t).

Simplify the expression: (5a² - ab - 3b²) - (6a² - 7ab + 10b²).

Simplify the expression: (15a² - 7ab - 12b + 3c²) - (8ab - 3b + 12c²).

Simplify the expression: (x² + y²) - (x + y²) + (12x² + y)

Simplify the expression: (1/2 x² + 1/3 x) + (2/3 x - 1/2) - (1/3 x² - 1/2).

Simplify the expression: 3x(x² - 5x + 4).

Simplify the expression: 7xy²(3x - 5xy⁴).

Simplify the expression: -3a⁴bc²(2a³b + 7abc⁵).

Simplify the expression: (x² + 3x + 5)(x + 2).