m2 trigonometric functions

1.suppose tan a and tan b are the roots of the equation t^2+6t-2=0, where a is an obtuse angle and b is an acute angle
(a) find the value of tan a - tan b using sum of roots and product of toos
my problem:why the answer is -2root11 but not 2root11?
2.it is given that sin x -sin y=p and cos x +cos y =q. expresscos(x+y) in terms of p and q
3. if cos a =1/2 and sin(a-b)=(root3)/2, find the value of tan b without using a calculator.
my problem:i cannot get root3 as the answer. i get -root36 and 0 only