Please help, with workings:

Practice Question $2$:

The expected returns, volatilities, and correlations among four stocks are

as follows:

Expected Returns

Volatilities & Correlations

Part A

Assuming equal weights for each stock, what are the standard deviations

for the following portfolios? $(12$ marks$)$

a$.$ A and $C$

b$.B$ and $C$

c$.$ C and D

d$.D$ and $A$