You know the first binomial formula: $(a+b)^2 = a^2 + 2ab + b^2$. That means you can expand the bracket to get the sum at the RHS but you also can get a squared bracket if you have a sum like the one at the RHS:
$\begin{array}{lcr}a^2+2 \cdot ab+b^2&=&(a+b)^2\\x^2 + 2 \cdot \frac{7}{4} x + \left ( \frac{7}{4} \right )^2 &=&\left ( x + \frac{7}{4} \right ) ^2\end{array}$