Some algebra is needed, y'all < 3

42 = piano and/or guitar
26 = guitar
33 = piano
19 = piano and guitar

piano only = 33 - 19
= 14

guitar only = 26 - 19
= 7

total played = 19 + 14 + 7
= 40

Total musicians = 42
not played = 42 - 40
= 2

the answer for first column under the "piano"
play guitar = 19
Don't play guitar = 14
Total = 33

the answer for second column under the "Don't play piano"
play guitar = 7
Don't play guitar = 2
Total = 9

the answer for third column under the "Total"
play guitar = 26
Don't play guitar = 16
Total = 42