Taking $q(x)=1$, $q(x)=x$ and $q(x)=x^2$ in equation (1) and
working out the integral (easy!) will give you three equations
which you can solve to find $\lambda_1,\ \lambda_2$ and
$\lambda_3$.
The key to showing that the same formula works for other
polynomials is to show that if it works for $1,\ x$ and $x^2$ it
works for any linear combination of them and so for any quadratic
polynomial.
Finally you can go on to check out the formula for $x^3,\ x^4$ and
$x^5$.