Find the minimum value of the following expression and the value of x between 0 and 360 when the minimum occurs .

3 cos 2x + sin 2x --- this is the expression

3 cos 2x + sin 2x = r cos(2x-a)

$\displaystyle r=\sqrt{10}$

tan a=1/3 , a =18.43

When minimum occurs , \sqrt{10}cos(2x-18.43)=-\sqrt{10}

cos(2x-18.43)=-1

2x-18.43=180 or 2x-18.43=270

Then i get my 2 values of x from there .

But i am wrong . Where is my mistake >?