(c+d)^3 is worded as <em>c plus d quantity cubed</em> which doesn't mean you cube everything inside. It means you multiply the quantity by itself 3 times.
(c+d)(c+d)(c+d)
<em>You can only multiply 2 at a time so do the first two then the result and the third one.</em>
<em>Distribute everything in one quantity to the other so </em><em>c*c = c^2 and c*d = cd</em>