5-t should be 5+t
Fragment of a discussion from Science talk:Math Exam Resources/Courses/MATH221/December 2011/Question 1 (a)
Wow! I need to watch out that code a bit more closely. Thanks for your carful work!
I've corrected the solution and added the line you asked. (I also checked in wolfram alpha and the solution is now correct)
David Kohler (talk)