Question 1003913
Points (t,h) for years, height.
Points are (4,9) and (8,12).

Try staying in slope-intercept form, {{{h=mt+b}}}.
{{{m=(12-9)/(8-4)}}} and {{{b=h-mt}}}.


Substitute either point, and once simplified in slope-intercept form for the described example, you have your vertical axis intercept, b.