c) \(\sqrt{x-4}-\sqrt{x+11}=-3\) (đk \(x\ge4\))
\(\Leftrightarrow\sqrt{x-4}+3=\sqrt{x+11}\)
\(\Leftrightarrow\left(\sqrt{x-4}+3\right)^2=x+11\)
\(\Leftrightarrow x-4+6\sqrt{x-4}+9=x+11\)
\(\Leftrightarrow6\sqrt{x-4}=6\)
\(\Leftrightarrow\sqrt{x-4}=1\)
\(\Leftrightarrow x-4=1\)
\(\Leftrightarrow x=5\)