Так как а - угол II четверти, то cos a < 0; tg a < 0; ctg a < 0
sin a = 3/5; cos a = -√(1 - sin^2 a) = -√(1 - 9/25) = -√(16/25) = -4/5
tg a = sin a / cos a = (3/5) : (-4/5) = -3/4
ctg a = 1/tg a = -4/3
sin 2a = 2sin a*cos a = 2*3/5*(-4/5) = -24/25
cos 2a = cos^2 a - sin^2 a = 16/25 - 9/25 = 7/25