Question 11973
I'm presuming that you have to "simplify" the given expressions.

Simplify:
 {{{7m(-3n) + 3m(-4n)}}} Apply the "distibutive property" (Check your math book if you are not familiar with this.)

First do:
 {{{7m(-3n)}}} Multiply the 7 and (-3) to get -21, then m times n = mn, finally, -21 times mn = -21mn
Repeat this for the second term:
{{{3m(-4n)}}} Multiply 3 times (-4) to get -12, then m times n = mn, finally, -12 timed mn = -12mn. You will now have {{{-21mn - 12mn}}} and when you add these two negative quantities you will get -33mn. 

{{{-21mn - 12mn = -33mn}}}

Second problem is handled in exactly the same way.

{{{(10m)(12n)}}} Multiply 10 by 12 to get 120, then m times n = mn, finally, 120 times mn = 120mn

{{{(10m)(12n) = 120mn}}}