The error happened when -3 was multiplied by -r in the first step, negative and negative makes a positive so it should have been -18 + 3r= 9 in the second step. Hope this helps :)
I'm pretty sure its 100 because if she gave each of her 19 students 5 invitations that equals 95. (19×5=95) And She had 5 leftover equals 100. (95+5=100)
