Determine the sum: (9r3s2t − 4r2s2t2 − 6rs2t3) + (−4r3s2t + 8rs2t3).
3r10s10t10
−r3s2t + 4rs2t3
5r6s4t2 − 4r2s2t2 + 2r2s4t6
5r3s2t − 4r2s2t2 + 2rs2t3