$${\mathtt{a}}{\mathtt{\,\small\textbf+\,}}{\frac{{\mathtt{2}}{\mathtt{\,\times\,}}{\mathtt{a}}}{{\mathtt{5}}}} = -{\mathtt{21}}$$
You can write this as
$$\\ \frac{5\times a}{5}+\frac{2\times a}{5}=-21\\\\\frac{7\times a}{5}=-21\\\\7\times a=-21\times 5\\\\a=\frac{-21\times 5}{7}\\\\a=-3\times 5\\\\a=-15$$
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