How to find gain and phase crossover from a Nyquist plot?
To use Matlab for checking the answer which was calculated by hand, try one of the below methods,

allmargin(system)
increase number of frequency points in the Nyquist plot by manually supplying a large number of frequency points. e.g. nyquist(system, [0:1e-2:1e3]). and then use the cursor. Newer versions of Matlab will show the frequency as well as other details from which you can manually calculate the margins
Use the option given in the context menu / right click menu to show the margins using the cursor / data label.

Example from Mathworks
Sample image from mathworks
Note: Remember that, to calculate margins, you plot the Nyquist for the open loop system and that too where the final system is transformed into a unity feedback configuration.
Note : To calculate by hand, use the conditions \$|G(s)| = 1\$ and \$\angle G(s) = \pi\$ to solve for the gain cross over and phase cross over frequency(ies) respectively. Here \$G(s)\$ is the open loop transfer function of the unity feedback system. Information needed for this is not given in the question.