Matboard Mat

more of these stupid problems. i get one and then cannot get the next?

A picture 16 inches by 25 inches is to be mounted on a piece of matboard so that there is an even amount of mat all around the picture. How wide will the border be if the area of the mounted picture is 792 square inches?

(16 +2x)(25+2x) = 792
400 +82x +4x^2 =792
4x^2 +82x -392=0
2x^2 +41x -196 =
(x-4)(2x+49) =0
Disregard negative answer
x=4

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