Then we may write x21+ x22= 2(y21 + y22), and hence n = x20+4(y21 +y22 +x23),
where we may assume that y1 and y2 have the same parity (by the pigeonhole principle), thus y21 +y22 = 2(z21 +z22).
It follows that n = x20+ 8(z21 + z22) + 4x23= x20+ 2(2z1)2 + 4x23+ 8z22, as required.