(2s - t^4)^7 = (2s)^7 - 7(2s)^6(t^4) + 21(2s)^5(t^4)^2 - 35(2s)^4(t^4)^3 + 35(2s)^3(t^4)^4 - 21(2s)^2(t^4)^5 + 7(2s)(t^4)^6 - (t^4)^7
= 128s^7 - 7(64s^6)(t^4) + 21(32s^5)(t^4)^2 - 35(16s^4)(t^4)^3 + 35(8s^3)(t^4)^4 - 21(4s^2)(t^4)^5 + 7(2s)(t^4)^6 - (t^4)7
= 128s^7 - 448s^6t^4 + 672s^5t^8 - 560s^4t^12 + 280s^3t^16 - 84s^2t^20 + 14st^24 - t^28