Prove that the length of the common chord of two circles

Prove that the length of the common chord of two circles x^2 + y^2 + 2hx + a^2=0 and x^2 + y^2) - 2ky - a^2=0 is 2 akar([((h^2 - a^2) (k^2 + a^2))/(h^2 + k^2)])