So first you plug 2 in for t
4(2.72)^(-2/2)
4/2.72 (because the simplified exponent is -1)
divide and that's your answer.
Next you plug 1 in for t
4(2.72)^(-1/2)
4/sqrt(2.72) (because -1/2 as an exponent is 1/sqrt(x))
plug that into a calculator and that's your answer
For c, you plug 1 in for V(t).
1=4(2.72)^(-t/2)
1/4=2.72^(-t/2)
Here is where I get stuck because I haven't done logarithms in a year and forget how to do them but I know that's what you have to do for this problem.
In the last problem you plug x in for V(t) and I'd assume go about solving it the same as c. Sorry I couldn't be of more help.