Math Help - prove that

if a cos^3 theta + 3 a sin^2 theta cos theta = m and a sin^3 theta + 3 a cos ^2 theta = nprove that (m+n)^2/3 + (m - n)^ 2/3 = 2a ^ 2/3

Start with

or is it ??

make the relevant substitutions for m and n from above then expand and simplify until you get the RHS