7cos (3x+13n/4) = - 2 решите прошу!

7cos (3x+13π/4) = - 2

7cos (3x+8π/4+5π/4) = - 2

7cos (3x+2π+5π/4) = - 2

7cos (3x+5π/4) = - 2

cos (3x+5π/4) = - 2/7

3x+5π/4=+-arccos (-2/7) + 2πn

3x+5π/4=+-arccos (2/7) + 2πn

3x=+-arccos (2/7) - 5π/4+2πn

x=+-arccos (2/7) / 3-5π/12+2πn/3