Answer
$m^{2}$+6mn+9$n^{2}$
Work Step by Step
There is one formula you can use in this case:
$(a+b)^{2}$=$a^{2}$+2ab+$b^{2}$
=> $(m+3n)^{2}$
=$m^{2}$+2(m)(3n)+$(3n)^{2}$ (Apply the formula above)
=$m^{2}$+6mn+9$n^{2}$ (Now, simplify)
