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According to additional details, the equation at the very top is incorrect, making your second line correct. This is what you should get then:
n(n+1)(n+2)(n+3)/(2*3*4) + (n+1)(n+2)(n+3)/(2*3)
= n(n+1)(n+2)(n+3)/(2*3*4) + (n+1)(n+2)(n+3)*4/(2*3*4)
= (n(n+1)(n+2)(n+3) + 4(n+1)(n+2)(n+3)) / (2*3*4)
= (n+1)(n+2)(n+3) (n + 4) / 24
= (n² + 2n + n + 2) (n² + 4n + 3n + 12) / 24
= (n² + 3n + 2) (n² + 7n + 12) / 24
= (n² (n² + 7n + 12) + 3n (n² + 7n + 12) + 2 (n² + 7n + 12)) / 24
= (n⁴ + 7n³ + 12n² + 3n³ + 21n² + 36n + 2n² + 14n + 24) / 24
= (n⁴ + 10n³ + 35n² + 50n + 24) / 24