We can assume t^2-4t = t(t-4) ne 0, i.e. t \ne 0 and t ne 4 so we can cancel the common denominator when we get them.
(t+4)/t+3/(t-4)=-16/(t^2-4t)
{(t+4)(t-4) + 3t}/{t(t-4)} = {-16}/{t(t-4)}
(t+4)(t-4) + 3t = -16
t^2 -16 + 3t = -16
t(t+3) = 0
t = 0 was ruled out at the start
t = -3 is the sole solution
Check:
{-3+4}/-3 + 3/{-3 -4} = -1/3 -3/7 = -16/21
-16/{(-3)^2 - 4(-3) } = -16/{21} quad sqrt