30=-12-7x help help help help

Acircle has a circumference of 28.36 units what is the diameter of the circle

Aball is dropped from a height of 10m above the ground. it bounce to 90% of its previous height on each bounce. what is the approximate height that the ball bounce to the fourth bounce?

50 will mark show all use the linear combination method to solve the system of equations. explain each step of your solution. if steps are not explained, you will not receive credit for that step. 2x+9y= -26 -3x-7y=13