I am confused with a calculation with finding the variance of a 2 asset portfolio return (asset a and b) from the Schweser material.

Part of the formula says o^2 (Ra) - with o representing standard deviation and Ra representing the expected return of asset A.

Yet, when I look at examples of this formula being used, it appears this term (listed above) is being substituted for the Variance of Ra. I dont understand how these two terms are equal/substitutable. Thanks in advance