How to simplify an exponential functions in MATLAB?
Symbolic math is included in MATLAB, but rarely used in signal processing. There are cases, nevertheless, when we wish to use symbolic math packages to simplify some expressions. One such alternative is also Wolfram-Alpha web application.
Anyway, apart from a misunderstanding, you indeed have a (simple) mathematical problem:
Given a complex number $z$ , you have
$$ z + z^* = 2 ~\mathcal{Re}\{ z \} $$
or a complex-valued function $f(t)$
$$ f(t) + f^*(t) = 2 ~\mathcal{Re}\{ f(t) \} $$
If you look at your function $g(t)$, you will see that it's a sum of a complex-valued function $f(t)$ and its conjugate $f^*(t)$, where
$$f(t) = 14.9 ~j~e^{j 31^o} ~ e^{j 2t} $$
and its conjugate
$$f^*(t) = - 14.9 ~j~e^{-j 31^o} ~ e^{-j 2t} $$
hence
$$g(t) = f(t)+f^*(t) = 2 \mathcal{Re}\{ f(t) \} $$
Then you can see that
$$\mathcal{Re}\{ f(t) \} = \mathcal{Re}\{ 14.9 ~j~e^{j 31^o} ~ e^{j 2t} \} =\mathcal{Re}\{ 14.9 ~e^{j (121^o +2t)} \} = 14.9 \cos(121^o +2t)$$
Then your $g(t)$ is:
$$ g(t) = 2 \times \mathcal{Re}\{f(t)\} = 2 \times 14.9 \cos(121^o +2t) $$