Answer:
540x⁴
Explanation:
You want the 3rd term of the expansion of (x +6)⁶.
Binomial expansion
The k-th term of (a +b)^n is ...
C(n, k)a^(n-k)b^k
where terms are numbered 0 to n.
Third term
The third term of (x+6)⁶ is the above expression with k=2:
C(6, 2)a⁴b²
Where a=x and b=6, this is ...
15x⁴6² = 540x⁴
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Additional comment
C(n, k) is the function that represents the number of combinations of n things taken k at a time:
C(n, k) = n!/(k!(n-k)!)
C(6, 2) = 6!/(2!·4!) = 6·5/(2·1) = 15