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Select the x-coordinate of the vertex of the parabola defined by the function f(x) = -7x2 + 3x + 5.

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2 Answers

the x value for the vertex of a parabola is -b/2a =-3/(-2*7) = 3/14
is the answer, the derivation made by my colleague Piyushi L is perfect as well.  
df(x) / dx should be 0 for the parabola f(x) at the vertex point
 
df(x)/dx = -14x + 3 = 0
 
x = 3/14
 
Answer is c