If $\overrightarrow a=(-1,1,1)\:\:\overrightarrow b=(2,0,1)\:and\:\overrightarrow x$ is vector such that $\overrightarrow x.\overrightarrow a=7,\:\overrightarrow x.\overrightarrow b=0$ and $\overrightarrow a,\overrightarrow b,\overrightarrow x$ are coplanar, then $\overrightarrow x=?$

Question

$(a)-3\hat i+4\hat j+6\hat k\:\:\qquad\:(b)\:\:3\hat i+16\hat j-6\hat k\:\:\qquad\:(c)\:\:-3/2\hat i+5/2\hat j+3\hat k\:\:\qquad\:(d)\:\:3\hat i+4\hat j-6\hat k$