REwarding Brainliest to correct Answer
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notice... the dog's pen perimeter, does not include the side that's bordering the garden's, since that side will use the heavy duty fence, instead of the light one
so, the sum of both of those costs, will be the C(x)
so, just take the derivative of it, and set it to 0 to find the extremas, and do a first-derivative test for any minimum