To multiply monomials, multiply the coefficients (the numbers that come before the variables) of each term, add the exponents of like variables, and multiply the different variables together.
\(4a \times 3ab+3 a^2 \times 9b\)
\(= 4 \times 3 \times (a \times ab)+3 \times 9 \times ( a^2b)\)
\(=12 a^2b+27a^2b\)
\(= 39a^2b \)