How do you factor this equation

I really, really don't get this stuff.

The equation is -3m^2+48n^2

The ^ stands for "to the power of" (since I can't actually make it go up...).

I know to make it:

-1(3m^2-48n^2)

To take out the negative coefficient, but I don't know what to after that. I'd do:

3(m^2-16n^2) and go from there, but I can't since I already did -1, can I? Like, can I do:

-1[3(m^2-48n^2)]

?

And if so, what do I do from there?!

I don't get this, at all.

You were correct as far as you went:
-1(3m^2-48n^2)
Now factor out the common factor of 3:
-3 (m^2-16n^2)
Since 16n^2 = (4n)^2 what remains in parentheses is the difference of squares, so use the rule for that:
-3 (m + 4n) (m - 4n)