Question 1056798
There is a formula for finding the peak when
the equation looks like 
{{{ f(x) = ax^2 + b*x + c }}}, it is:
{{{ x[max] = -b/(2a) }}}
{{{ x[max] = -6/(2*(-.01)) }}}
{{{ x[max] = 6/.02 }}}
{{{ x[max] = 300 }}}
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You can also find the derivative of {{{ P(x) }}}
and set it to zero to find the peak.
The derivative of {{{ P(x) }}} is:
{{{ -.02x + 6 = 0 }}}
{{{ -.02x = -6 }}}
{{{ x = 6/.02 }}}
{{{ x = 300 }}} 
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Selling 300 telephones maximizes profit