I'm pretty sure you are asking for the value of b. Let's actually write it and then solve it:

Solving for b yields:

So,
b = 19.59. If you want the answer in simplified radical form, we can also do that:

So,
b = 8√6 = 19.59
His score is 50
How to:
First test grade:0
Second test grade:50
Third test grade: 100
Add all three and divide by 3
Answer:
Step-by-step explanation:
Sum
-2d + 1 + 6d + 4
Solving like terms
4d + 5
Difference
-2d + 1 - 6d - 4
-8d - 3
Part A:
y=mx+b
621=83(7)+b
Part B:
621=83(7)+b
621=581+b
621-581=b
b=40
Insurance cost is 40$
<span>You need to set up 2 equations for this, a system to be exact
let x represent the cheaper phone and y represent the upgraded phone
for store costs you will have
5x+25y=200
for selling prices you will have 30x+100y=900
set these two equations together
5x+25y=200
30x+100y=900
multiply the first one by -4 and add them
-20x-100y=-800
30x+100y=900
10x=100
x=10
10 cheaper phones, plug into any equation again
5(10)+25(y)=200
y=6<<<6 upgraded phones</span>