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Let the width be x meters; then the length is (x+6) meters.
The perimeter is the sum of four lengths
P = x + (x+6) + x + (x+6) = 2x + 2*(x+6), or
24 = 2x + 2*(x+6).
You may write it in this form
x + (x+6) = 24/2
x + (x+6) = 12.
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I STOP at this point, since the assignment does nor requires to solve the equation.
The person, familiar with the subject, who just solved such problems many times, writes the equation
x + (x+6) = half of the perimeter
without any explanations, assuming that everything is just crystally clear.