The image is really unclear but I will assume that the number of teams given = 66
and the formula is
[tex]n = \frac{ {t }^{2} - t }{2} [/tex]
plug in the number of teams
[tex]66 = \frac{ {t}^{2} - t }{2} [/tex]
multiply both sides by two
[tex]132 = {t}^{2} - t[/tex]
subtract 132 from both sides and factor
t = 12
