You square a sum term by term.
$(a+b+c)^2$ is the three squares plus every cross product: $a^2+b^2+c^2+2ab+2ac+2bc$. The vector version is the same: $|\underline{a}+\underline{b}|^2 = |\underline{a}|^2 + 2\,\underline{a}\cdot\underline{b} + |\underline{b}|^2$.
Next time: Count how many terms the square should have before writing them.