Evaluate
\left(3x+k\right)^{2}+x^{2}
Expand
10x^{2}+6kx+k^{2}
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x^{2}+k^{2}+6kx+9x^{2}
Use binomial theorem \left(a+b\right)^{2}=a^{2}+2ab+b^{2} to expand \left(k+3x\right)^{2}.
10x^{2}+k^{2}+6kx
Combine x^{2} and 9x^{2} to get 10x^{2}.
x^{2}+k^{2}+6kx+9x^{2}
Use binomial theorem \left(a+b\right)^{2}=a^{2}+2ab+b^{2} to expand \left(k+3x\right)^{2}.
10x^{2}+k^{2}+6kx
Combine x^{2} and 9x^{2} to get 10x^{2}.
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