How to put character 'B' on column 8 row 5 in text mode?

This is what I know:

text-mode 80x25 characters. I need to put character 'B' on column 8, row 5. What's the formula to know this answer?

EDIT:

How would I do this for graphical mode? For example:

resolution of 1024x768

yellow pixel on (123,456)

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