How to find the area and circumference of a circle given the radius or diameter

Steve wants to develop a way to find the total cost of any item including sales tax. the sales tax in his city is 6%. his friend jenna writes the expression x + 6x where x is the cost of the item. explain whether her expression is correct based on the situation.

Simplify 15(x - 4)(x -1)-3(x + 4)(x - 1) with steps

Ineed to know the answer to this algebra question.