\[\sqrt{x+15}+ \sqrt{x}=15\]
.We can rearrange this to give
\[\sqrt{x+15}= \sqrt{x}+15\]
.Now square both sides.
\[x+15= x+30 \sqrt{x} +225\]
.Now
\[x\]
cancels from both sides, to give \[15= 30 \sqrt{x} +225 \rightarrow x = (\frac{15-225}{30})^2=49\]
.