#2m# means (#2xxm#)

One way to think of this is having

#color(white)("XXXX")#2 rows each with the same number of monkeys

#color(white)("XXXX")##color(white)("XXXX")#(we're not sure of the exact number of monkeys in each row,

#color(white)("XXXX")##color(white)("XXXX")#but we can see that there are the same number

#color(white)("XXXX")##color(white)("XXXX")#and for convenience we will call this number #m#)

Similarly #5m# means (#5xxm#)

And we can think of this in the same terms as #2m#

#color(white)("XXXX")#5 rows each with the same number of monkeys (and the same number of monkeys as in the 2 rows)

If we add 2 rows of monkeys and 5 rows of monkeys

we get 7 rows of monkeys (each row with #m# monkeys)

We can write this in symbols as

#color(white)("XXXX")##2m + 5m = 7m#