a = 3, b= - 4 and c = - 4
expand the left side using FOIL
(2x + 1)(ax + b) = 2ax² + 2bx + ax + b = 2ax² + x(2b + a) + b
compare the coefficients of expressions on left and right sides.
compare 2ax² + x(2b +a) + b with 6x² - 5x + c
coefficients of x² terms → 2a = 6 ⇒ a = 3
coefficients of x terms → 2b + a = - 5 → 2b + 3 = - 5 → 2b = - 8 ⇒ b = - 4
constant terms c = b = - 4