You don't plug in t = 5 again. We don't know the value of t yet. We're trying to solve for it.
We do know u(t) is 135 as we want the temperature to be 135 degrees. Plug this in and isolate t.
u(t) = 70+380e^(-0.1004183888t)
135 = 70+380e^(-0.1004183888t)
135-70 = 380e^(-0.1004183888t)
65 = 380e^(-0.1004183888t)
380e^(-0.1004183888t) = 65
e^(-0.1004183888t) = 65/380
e^(-0.1004183888t) = 13/76
-0.1004183888t = ln(13/76)
-0.1004183888t = -1.7657839828248
t = -1.7657839828248/(-0.1004183888)
t = 17.5842692152893
So it takes about 17.58 minutes for the item to cool to the desired temp
because quadrilatural (idk how to spell it) means somthing that is not 3d or like, equal sides (or somthing like that you know what i meanz)
hope this halps!