First: Please use "^" to denote exponentiation: 3x(8x^2 - 7x + 2).
Now apply the distributive property of multiplication: Mult. each of the 3 terms inside the parentheses by 3x: 24x^3 - 21x^2 + 6x. Please note that this is not a solution, since you're not working with an equation; it's a simlified expression.