$P=\left[M L^{-1} T^{-2}\right]$
$a$ will have same unit as $t^{2} .$ So, $a=\left[T^{2}\right]$
$b=\left[\frac{a-t^{2}}{p x}\right]=\frac{\left[T^{2}\right]}{\left[M L^{-1} T^{-2} L\right]}=\left[M^{-1} L^{0} T^{4}\right]$
$\frac{a}{b}=\frac{\left[T^{2}\right]}{\left[M^{-1} L^{0} T^{4}\right]}=\left[M L^{0} T^{-2}\right]$