3. Find the exact values of sin 2 θ and cos 2 θ for cos θ= 6/13 on the interval 0 degree < θ < 90 degree.

Equation 

y = 2x + 6 (1)
3x - y = 6 (2)


Substitute equation (1) into (2)

3x - (2x + 6) = 6


Remove bracket

3x - 2x - 6 = 6


Combine like terms

x - 6  = 6


Add 6 to both sides

x = 6 + 6
x = 12


Substitute x = 12 into equation (1)

y = 2 (12)  + 6
y = 24 + 6
y = 30


Answer : x = 12, y = 30