yes, what you want is to find A(10) when A_0 = 4. now plug in the value for a and solve

you need to know what each term is. that way, you can translate from the word problem to the math equation and know what they are asking you for and what you are given.

is the amount left after a time has passed

is the initial amount. this is what we have when time is zero. nothing has started to decay yet

is the the amount of time that has passed.