What is the coefficient of x^4 in the expansion of (x+3)^7

Start by expanding (a+b)⁷, using Pascal's Triangle or the binomial coefficient function to work out the coefficients:a⁷ + 7a⁶b + 21a⁵b² + 35a⁴b³ + 35a³b⁴ + 21a²b⁵ + 7ab⁶ + b⁷ , where a=x and b=3As the question wants the coefficient of x³, we need to look for a³ . The expansion gives 35a³b⁴, so we must substitute values in for a and b. As stated earlier, a=3 and b=3, hence; 35a³b⁴ = 35 * x³ * 3⁴ = 35 * 81 * x³ = 2835x³Therefore the coefficient of x³ is 2835