2x - 1 must be a factor of the given polynomial.
One of the roots must be x = 1/2
Plug in x = 1/2, and equate the polynomial to zero:
2(1/2)^3 + a(1/2)^2 + 11(1/2) + a + 3 = 0
1/4 + a/4 + 11/2 + a + 3 = 0
5a/4 + 35/4 = 0
5a + 35 = 0
a = -7
SOLUTION: 2x^3 - 7x^2 + 11x - 4 is exactly divisible by 2x - 1
Check: (x^2 - 3x + 4)(2x - 1) = 2x^3 - 7x^2 + 11x - 4