2(2cosx+sinx)=-(cosx-7sinx)
4cosx+2sinx+cosx-7sinx=0
5cosx-5sinx=0
cosx-sinx=0
sinx-sin(π/2-x)=0
2sin(x-π/4)cosπ/4=0
sin(x-π/4)=0
x-π/4=πn
x=π/4+πn
