The system:
3 x + 4 y = 40
x + 6 y = 18,
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x = 18 - 6 y
3 ( 18 - 6 y ) + 4 y = 40
54 - 18 y + 4 y = 40
54 - 40 = 18 y - 4 y
14 = 14 y
y = $1
x = 18 - 6
x = $12
Answer: Each DVD costs: $12.
When n = 1, m = -5.5, when n = 2, m = -8.5, when n = 3, m = -11
f(x)=3x^2-18x+10
a = 3, b = -18 and c = 10
The vertex of a parabola is in form a(x+d)^2 + e
d = b/2a = -18/2(3) = -18/6 = -3
e = c-b^2/4a = 10 - -18^2/4(3) = 10-27 = -17
Now the vertex form of the parabola becomes 3(x-3)^2 -17
Use the vertex form of the parabola in the vertex form of y = a(x-h)^2 +k
Where a = 3, h = -3 and k = -17
Now you have y = 3(x-(-3))^2 +(-17)
Simplify: y = 3(x+3)^2 -17
The vertex becomes the h and k values of (3,-17)