To express the given function in the form #A cos (omega t + phi)#, notice that according to the trigonometric sum rule
#A cos (omega t + phi) = A [cos (omega t )cos (phi)-sin(omega t)sin(phi)] = -A sin(phi) sin(omega t) + A cos(phi)cos(omega t)#
Comparing with the expression given, we have #omega = pi# nd
#A sin(phi) = -1.2, qquad A cos(phi) = 3.5#
Thus
#A^2 = = (A sin(phi))^2 + (A cos(phi))^2= (-1.2)^2+(3.5)^2 = 13.69 = 3.7^2#
So, #A = 3.7#
Now,
#tan(phi) = {A sin(phi)}/{A cos(phi)} = -{1.2}/{3.5} = -0.3429#
Since #sin(phi)# is negative while #cos(phi)# is positive, the angle #phi# must be in the fourth quadrant. So
#phi =2pi-tan^{-1}(12/35)=5.953 #