You need to use the binomial formula, such that:
`(x+y)^n = sum_(k=0)^n ((n),(k)) x^(n-k) y^k`
You need to replace 7a for x, b for y and 3 for n, such that:
`(7a+b)^3 = 3C0 (7a)^3+3C1 (7a)^2*(b)^1+3C2 (7a)^1*(b)^2+3C3(b)^3`
By definition, nC0 = nCn = 1, hence `3C0 = 3C3 = 1.`
By definition nC1 = nC(n-1) = n, hence `3C1 = 3C2 = 3.`
`(7a+b)^3 = 343a^3 + 147a^2*b+ 21a*b^2+b^3`
Hence, expanding the complex number using binomial theorem yields the simplified result `(7a+b)^3 = 343a^3 + 147a^2*b+ 21a*b^2+b^3.`
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