Okay so I did the dot product with a and b and I got (2,-10,-1).

Then: √(2)^2+(-10)^2+(-1)^2) and I get

A = √(105). Am I correct?

Did I do it right?

You mean the vector cross product!

\(\begin{array}{|rcll|} \hline A &=& \sqrt{2^2+(-10)^2+(-7)^2} \\ A &=& \sqrt{4+100+49} \\ \mathbf{A} & \mathbf{=} & \mathbf{\sqrt{153}} \\ \hline \end{array}\)