Is this a typo in Chatper: Convolution

Is this a typo in the convolution. It seems like the signs are reversed. I have attached images to show the kernel and the sample matrix. Should the equation be -a1-a2-a3+a7+a8+a9? Unless I am betting something backwards.
Thanks :pray::smile:

Here is the link to the chapter.

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I think you’re right. I’m reading through the book right now and I noticed the exact same thing.