f(t)= - cos (pi/4 - 3t)
Use what you know
first its a cosine wave so its a periodic squiggley--its a cosine squiggly so at 0 its 1 and at pi its 0. The midpoint is1
second is a negative cosine squiggley--so the normal cosine squiggley is upside down
third it has a 3 term in the variable t, so the frequency is tripled , normal wavelength of a cosine squiggley is 2pi, but if 3t=2pi, then wavelength is 2pi/3.
fourth its been displaced at -cos(0)=-1 0=pi/4-3to so to=pi/12 so the t axis is shifted to the right by pi/12
fifth the cos is negative--since cos is a even function the cosine wave is reflected about the to line (i.e. do nothing past step 4)