Đáp án:

$\begin{array}{l}
1)D\left( {x;y} \right) \Rightarrow \overrightarrow {DC}  = \left( {2 - x; - y} \right)\\
\overrightarrow {AB}  = \overrightarrow {DC} \\
 \Rightarrow \left( {4;6} \right) = \left( {2 - x; - y} \right)\\
 \Rightarrow \left\{ \begin{array}{l}
4 = 2 - x\\
6 =  - y
\end{array} \right. \Rightarrow \left\{ \begin{array}{l}
x =  - 2\\
y =  - 6
\end{array} \right.\\
 \Rightarrow D\left( { - 2; - 6} \right)\\
2)\\
a)\overrightarrow u  + 3\overrightarrow v \\
 \Rightarrow \left\{ \begin{array}{l}
x = 5 + 3.3 = 14\\
y =  - 2 + 3.1 = 1
\end{array} \right.\\
 \Rightarrow \overrightarrow u  + 3\overrightarrow v  = \left( {14;1} \right)\\
b)2\overrightarrow u  + 5\overrightarrow v  = \left( {25;1} \right)\\
3)\sqrt {2x + 1}  = 3\left( {dk:x \ge  - \frac{1}{2}} \right)\\
 \Rightarrow 2x + 1 = 9\\
 \Rightarrow x = 4\left( {tm} \right)
\end{array}$