we will use the double angle formulas for cosine, these are:

cos(A + B) = cosAcosB - sinAsinB

cos(A - B) = cosAcosB + sinAsinB

cos(pi - t) = cos(pi)cos(t) + sin(pi)sin(t) = (-1)(4/5) + 0 = -4/5cos t = 4/5

a) cos(pi - t)

cos(t + pi) = cos(t)cos(pi) - sin(t)sin(pi) = (4/5)(-1) - 0 = -4/5cos t = 4/5

b) cos(t + pi)