The dominant mode of a rectangular waveguide is the TE1,0 mode, with m=1 & n=0

$F_c$ = $1.5*10^{8}{\sqrt{({\frac{m}{a})^2}+ ({\frac{n}{b}})^2}}$

$F_c$ = $1.5*10^{8}{\sqrt{({\frac{1}{0.050})^2}+ ({\frac{0}{0.025}})^2}}$

$F_c$ = 3 GHz