First, organize your variables:
Let x= the number of small packages and y=the number of large packages. C= the cost of sending the packages.
Set up your equation:
2.10x+3.90y=C
Substitute 1 and 5 for x and y since that's how many packages Diane wants to mail, and then solve for C
2.10(1)+3.90(5)=C
2.10+19.50=C
21.60=C
It will cost Diane $21.60