Substitute f(x) = x^2 + a x + b into the integral equation and compare the coefficients.
Then we obtain two linear equations in a and b:
a + b = - 2/3
3a + 2b = - 8/3
which yield a = - 4/3 and b = 2/3.
Answer: f(x) = x^2 - (4/3) x + 2/3.
Substitute f(x) = x^2 + a x + b into the integral equation and compare the coefficients.
Then we obtain two linear equations in a and b:
a + b = - 2/3
3a + 2b = - 8/3
which yield a = - 4/3 and b = 2/3.
Answer: f(x) = x^2 - (4/3) x + 2/3.