Factor t12 – r3s15.

(t4 – rs5)(t8 – rs5t4 – r2s10)

(t4 – rs5)(t8 + rs5t4 + r2s10)

(t9 – rs12)(t18 + rs12t9 + r2s24)

(t4 + rs5)(t8 – rs5t4 + r2s10)