y = (x +12)(x +8)
or
y = x² +20x +96
If the current sign is x inches long, adding 12 inches to the length makes it ...
... length = x + 12
If the current sign is x inches tall, adding 8 inches to the height makes it ...
... height = x + 8
The area of the sign is the product of length and height:
... y = (x +12)(x +8)
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This equation is certainly adequate for finding the area, but some may prefer it in expanded form:
... y = x(x +8) +12(x +8) = x² +8x +12x +96
... y = x² +20x +96