What is the highest common factor of $3$3 and $15x$15x?

What is the highest common factor of $6x$6x and $21$21?

Factor the following expression by taking out the highest common factor:

$7x+35$7x+35

$48+12y$48+12y

Factor polynomial expressions involving common factors, trinomials, and differences of squares, using a variety of tools and strategies