How do you use units and convert them to understand and solve problems?

A unit is a measurement. for example, a unit could be an inch, or it could be a foot. to convert you need the exact measurement so if a line is 12 inches it's 1 foot. if it is 13 then it stays in inches. You convert units to the easiest variable to calculate and then you solve your equation from there. 

Hope I helped! :)

The first one is correct

B. because the 7 represents "seven times", x represents "a number", 12 represents "plus twelve", and -4 represents "minus four"