What is 3mn x 4m2n?

To multiply the expressions (3mn) and (4m^2n), you multiply the coefficients and then combine like terms. The coefficients (3) and (4) multiply to give (12), and for the variables, you add the exponents of like terms: (m^{1+2} = m^3) and (n^{1+1} = n^2). Therefore, the result is (12m^3n^2).