Are you dividing 2t+1 by t alone or by t+3? If by t+3, you must enclose (t+3) inside parentheses:
f(t) = (2t+1) / (t+3)
If this is the correct function, then decide what kind of function it is, so that you can choose the correct rule of differentiation. It's a QUOTIENT.
Please look up, jot down and then follow the Quotient Rule.
The numerator here is (2t+1), and the denominator is (t+3).