$3-$ In the shown simple model, the two solid steel $(E=200$GPa,$v=0\mathrm{.}3)$ shafts

are connected to a coupling disk at $B$ and to fixed supports at A and $C.$ Find:

i$)$ The minimum required yield strength of the used steel if no point passes

yield. Use the Tresca criterion for yield analysis.

ii$)$ The angle of twist at $B.$

I am getting stuck when I have to convert the units when doing matrix multiplication Ax$=$b $-->$ A$^-1$b$=$x