Question 1090563
{{{R(x)=(30-5x)x}}}
{{{R(x)=-5x^2+30x}}}
Complete the square,
{{{R(x)=-5(x^2-6x)}}}
{{{R(x)=-5(x^2-6x+9)+5(9)}}}
{{{R(x)=-5(x-3)^2+45}}}
So the revenue function is in vertex formula. 
The maximum of {{{45}}} occurs when {{{x=3}}}.