A charged particle of unit mass and unit charge moves with velocity of $\vec{v}=(8\hat{i}+6\hat{j})$ m/s in a magnetic field of $\vec{B}=2\hat{k}T$. Then:

$r=\frac{mv}{qB}$
$=\frac{\sqrt{8^2+6^2}}{2}$
$=5$
Both $x^2+y^2$ and $x^2+y^2-4x-21=0$ are possible because Magnetic field is towards the positive z axis . 
$T=\frac{2\pi m}{qB}$
$=\frac{2\pi }{2}$
$=\pi$
Particle will move in x-y plane with radius equals to $5$