Definite Integration

Let f : R \(\to\) R be a continuous function satisfying f(x) + f(x + k) = n, for all x \(\in\) R where k > 0 and n is a positive integer. If \({I_1} = \int\limits_0^{4nk} {f(x)dx}\) and \({I_2} = \int\limits_{ - k}^{3k} {f(x)dx}\), then :