Question 681777
s(t) = −16t2 + vt + a.
If s(1) = 99 and s(4) = 129, what are v and a? 

99= -16(1)^2+v(1)+a
99=v+a-16
v+a=99+16
v+a=115..........(1)

129= -16(4^2)+4v+a

129=-256+4v+a

4v+a=129+256
4v+a=386...........(2)

Solve (1) & (2) 

subtract (2) from (1)
-3v=-276
v=92
Plug v in equation above to get a