Answer:
Mr. Sato needs 9 more magnets.
Step-by-step explanation:
The 24 students will form 12 pairs. The total number of magnets needed is 12*3 = 36. Mr. Sato gave out 9*3=27 magnets already and so he will need 36 - 27 = 9 more magnets.
Answer:

With the vertex 
We see that b = -10 and a = 2 and then the vertes wuld be:

And the best option is:
b. (-10,2)
Step-by-step explanation:
For this problem we have the following function:

And if we compare this expression with the general expression for a parabola given by:

With the vertex 
We see that b = -10 and a = 2 and then the vertes wuld be:

And the best option is:
b. (-10,2)
What?
The slope can be found between two points by finding the change in values divided by the change in values.