This problem is about using the right order of operations. To do that we can recur to PEMDAS, which means Parenthesis, Exponents, Multiplication-Division, Addition-Subtraction, that's the order to solve this problem.
First, we solve parenthesis using the distributive property:
Second, we solve like terms, that is, those terms which have variables, and those terms without variables.
Therefore, the answer is
27x+21, that's the simplest form to the given expression.