The measure of angle θ is . the measure of its reference angle is

The measure of angle θ is 7π/6 
The measure of its reference angle is pi/6 

And sin θ is -1/2 

Hope it helped :) 

Let
x----------------> width of the margin
we know that

[Area printed matter]=140
[Area printed matter]=[(14-2x)*(18-2x)]
then 
[(14-2x)*(18-2x)]=140--------> (252-28x-36x+4x²)=140
(252-64x+4x²)=140----------> 4x²-64x+112=0

using a graph tool---------> I solve the quadratic equation
see the attached figure
x1=2
x2=14----------> is not solution

the answer is 
the margin should be 2 cm