When you have an equation, such as the above, #w(t)# is saying that #w# is a function of #t#, real numbers.

So, when asked to find #w(-7)#, the question is asking,
At #t = -7#, what is the value of the function #w#? Therefore, if you substitute the value #-7# for each t into the equation, then #w = 15#.