You need to decide whether you are doing array indexing, or are creating a formula.
t=0:0.06:3;
fa(t)=10*exp(-5*t)
is interpreted as array indexing. Using () indexing on the left hand side of an assignment is almost always interpreted as array indexing.
The formula form would be
t=0:0.06:3;
fa = @(t) 10*exp(-5*t);
plot(t, fa(t))
The major exception is if you are using a symbolic variable, then
syms t
fa(t)=10*exp(-5*t)
creates a formula, which you then apply like
T=0:0.06:3;
plot(T, fa(T))
But easiest is usually what Alexander shows,
t=0:0.06:3;
fa=10*exp(-5*t)
plot(t,fa)
which is pure expression form.
