Answer :

pepe11
Sin(3×+13)=sin(pi/2-4×)
3x+13 =pi/2 -4×          or 3×+13 =pi-pi/2 + 4×
7× = pi/2 - 13 +k2pi   or -× = pi/2 +k2pi
× = pi/14 - 13/7 k2pi/7   or X = -pi/2 + kpi

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