Helena lost her marbles. but then she found them and put them in 4 bags with m marbles in each bag. she had 3 marbles left over that didn't fit in the bags. how many marbles does helena have in all?

For this case, the first thing we must do is define a variable:
 m = number of marbles
 We have then:
 "put them in 4 bags with m marbles in each bag"
 mathematically we have:
 4m
 Then,
 "She had 3 marbles left over that did not fit in the bags"
 mathematically we have:
 4m + 3
 
 Helena has:
 4m + 3 marbles.