Question 1118152
RT=D for R rate, T time, D distance.

Your example uses  R=x-5, and T=2x, and D=12.


{{{highlight_green(12/(2x)=x-5)}}}


{{{6/x=x-5}}}


{{{x^2-5x=6}}}


{{{x^2-5x-6=0}}}

{{{(x-6)(x+1)=0}}}

{{{highlight_green(x=6)}}}

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Speed was {{{highlight(1)}}} kilometer per hour.

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check:
recall, x=6.
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{{{12/(2x)=x-5}}}

{{{12/(2*6)=6-5}}}

{{{12/12=1}}}

{{{1=1}}}

(speed given as x-5;
which is evaluated as 1 )