1. ## 2sec²θ-7=0

2. $2\sec^2\theta-7=0$

$\frac{2}{\cos^2\theta}-7=0$

$\cos\theta=\pm\sqrt{\frac{2}{7}}$

Can you take it from here ?

3. $2sec^2 \theta - 7 = 0$
$sec^2 \theta = \frac{7}{2}$
$cos^2 \theta = \frac{2}{7}$
$cos \theta = \pm\frac{\sqrt{14}}{7}$