The approach by defining a generic function before choosing the variable values which Cornel showed sure has its benefits - especially when you want to experiment with various different input data.
Word of caution: You defined a variable "s" for displacement. So when you later type "s" Prime assumes you mean that variable. If you want it to be the unit seconds you have to tell that Prime by manually labelling that s as being a unit (by default you can see that as a unit is printed in dark blue and bold).
Its less awkward if you avoid using a variable name which also is the name of a unit.
Here are some ways to solve for a without having to define a generic function. I sure would advise NOT to use the first method (symbolic "solve").
Prime 9 sheet attached
