Whats another way to write -(-9) ?

The system of equations graphed below has how many solutions? y = 2x + 2

Dylan and dusty plan to take weekly surfing lessons together. if the 2-hour lessons are $20 per person and they plan to spend $100 each on new surfboards, what is the maximum number of lessons the two can take if the total amount spent for lessons and surfboards is at most $480?

What unit of been sure would be appropriate for the volume of a sphere with a radius of 2 meters