Please provide excel formula $1$

You own a stock portfolio invested $15$ percent in Stock Q$,20$

percent in Stock R$,30$ percent in Stock S$,$ and $35$ percent in

Stock T$.$ The betas for these four stocks are $\mathrm{.}79,1\mathrm{.}23,1\mathrm{.}13,$ and

$1\mathrm{.}36,$ respectively. What is the portfolio beta?