What do you reasonably expect to happen?
t varies over the interval -10 and 400. What is exp(-10)? If you don't know, then try it.
At the other end of that interval, what is exp(400)?
So on the y axis, you are hoping to plaot a curve that varies over many hundreds of powers of 10. What will that curve look like when you plot it? Do you want to guess?
At the same time, you want to plot the function exp(-t). So what does that curve do at the end points?
So this curve varies also over many hundreds of powers of 10, but it never grows lareger than roughly 22000.
How about exp(2*t)?
That one is even more extreme, since at the high end, it exceeds that largest number MATLAB can represent in double precision, a little larger the 10^300.
Now look at the y axis in the plot you generated. At the top left corner of that plot, do you see the x10^307? That tells you the y axis is scaled by that power of 10.
So seriously, what do you expect to see in that figure? MATLAB does its best to plot what you told it to plot, all in one figure. But remember the exponential function grows very rapidly. In fact, it grows exponentially rapidly.
Instead, you might have chosen a very much narrower range for t.