Could you pleaseUnderstand this so I can do it on my own and how do I do it in a calculator

O'Brien Inc. has the following data: $r_{RE}=5\mathrm{.}00\%$;$R{P}_M=6\mathrm{.}00\%$; and $b=1\mathrm{.}25.$ What is the firm's cost of equity from retained earnings based on the CAPM?

a$.13\mathrm{.}75\%$

b$.11\mathrm{.}88\%$

c$.12\mathrm{.}25\%$

d$.9\mathrm{.}63\%$

e$.12\mathrm{.}50\%$