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How did they find these position specific scoring matrix scores?

My lecture notes in Bioinformatics try to explain how to work out position specific scoring matrix, or position weight matrix. They use the following example in the notes. I understand the formula but I don't see how they could calculate -2.94 for a frequency/base count of 0, as log2(0) isn't a number. Could somebody explain how they might have found -2.94 for the 0 base counts? I am also not sure what pb value they used. I assume fbi means the base count (frequency) Image here of their explanation

sequence alignment sequencing
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updated by Mensur Dlakic Mensur Dlakic • written by jbrody11 • jbrody11

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Zero counts in sequence alignments is a common occurrence for biological sequences. Small pseudocounts are added to avoid numerical issues - see here. In practical terms this means that each residue will have a non-zero frequency.

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written by Mensur Dlakic Mensur Dlakic

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