Pls help me, I really dont understand

Ijust need these 2 questions answered (the second pic is just confirmation i'm not confident in that answer)

The median am of triangle ∆abc is half the length of the side towards which it is drawn, bc . prove that triangle ∆abc is a right triangle, and find angle mac and abm

Miguel has 334 baseball cards and 278 football cards. he says, i have 612 cards in all. is that reasonable? explain using the words round and estimate.