Find a number which decreased by 21 equals twice the opposite of the number

Because of your favorite lemonade it is $3.84 for 3 gallons. write this as an unit rate

Offering 30 if a plus b plus c equals 68 and ab plus bc plus ca equals 1121, where a, b, and c are all prime numbers, find the value of abc. the answer is 1978 but i need an explanation on how to get that.

Asquare picture with sides of 32 inches is to be mounted on a square mat with sides of 52 inches. find the area of the mat that will remain uncovered by the picture.