Find at least three nonzero terms including min two cosine a

Find at least three nonzero terms (including min. two cosine and min. two sine terms if they are all not zero) for the given function, and sketch the series.

f(x)= {[-6x, -pi (less than or equal to) x < 0] [6x, 0 (;ess than or equal to) x < pi]

f(x)= {[-6x, -pi (less than or equal to) x < 0] [6x, 0 (;ess than or equal to) x < pi] => x = { -pi, 0, pi}