For teacher reference:

                 TOP ROW: index number
                 MIDDLE ROW: correct letter
                 BOTTOM ROW: y-intercept

 
1  2   3 4    5  6   7      8   9  10 11   12   13 14      15 16 17  18 19  20
G  I   V E    M  E   A      F   I  S  H    I'   L  L       E  A  T   F  O   R
6 -18 -5 4    12 -22 0      -21 8 -8  7    -18  11 -1 5    4  0 -7   5  -12 17 

21   22 23  24   25 26 27 28 29   30 31   32  33    34  35  36 37   38 39 40
A    D  A   Y    T  E  A  C  H    M  E    T   O     F   I   S  H    I' L  L
-26  3  -26 -2   -7 4  0  2  -19  12 4    -7  14    -21 -18 18 7    8  11 -15

41 42 43   44 45  46   47   48 49 50 51  52 53 54 55  
E  A  T    F  O   R    A    L  I  F  E   T  I  M  E
4  0  -7   5  -12 -9   -26  11 8  5  -22 -7 8  12 -22