If you would like to know how many hours will Lena need to work before she can afford to buy the computer, you can calculate this using the following steps:
$900 - $473 = $427
$427 / $7 = 61 hours of yard work
Result: Lena will need to work 61 hours before she can afford to buy the computer.
You know angles B and C are both 60°, the values of the angles in an equilateral triangle. Since the sum of angles in a quadrilateral is 360°, you have
B + C + x + y = 360°
60° +60° + x + y = 360°
x + y = 240°
