In the arthmetic sequence, the first term is 3 and the fourth term is 12. Find the common difference (d) and the sum of the first 10 terms.

a. Finding du_n = u₁ + (n-1)du₄ = u₁ + (4-1)d12 = 3 + 3dd = 3
b. Sum of first 10 termss_n = n/2(2u₁ + (n-1)d)s₁₀ = 5(6 + 9d)s₁₀ = 30 + 45(3)s₁₀ = 165