\Ia
is not a LaTeX function. Neither is \ointM
. You probably want to put "or" into text mode, not math mode. You are probably looking for something like: "$Ia = \sum_{i=1}^\infty\oint M r{Q}^{2}\theta \,dA\; \text{or} \begin{Bmatrix} F = 12\, \mathrm{N}N \ \tau = 26\, \mathrm{N\cdot m}\end{Bmatrix}$"