Answer
$$11m-1$$
Work Step by Step
Like terms must (1) be raised to the same power and (2) consist of the same variable (i.e. x and y are not like terms). It is also important to note that numbers are all like terms with each other when not multiplied by anything. Thus, using the distributive property and combining like terms, we obtain:
$$7m-21+4m+20 \\ 11m-1$$
