Answer
a) $WXST TSPPYXI$
b) $NOJK KJGGPODJI$
c) $QHAR RABBYHCAJ$
Work Step by Step
(a)
f(p) = (p+4) mod 26
Let us apply the function to 1819 1415 15 14 11 11 20 19 8 1413
22 23 18 19 1918 15 15 24 23 12 18 17
Let A=0, B=1, C=2, D=3, E=4, F=5, G=6, H=7, 1=8, J=9, K=10, L=11
M=12, N=13, 0=14, P=15, Q=16, R=17, S=18, T=19, U=20, V=21
W=22, X=23, Y=24, Z=25
WXST TSPPYXI
(b)
f(p) = (p + 21) mod 26
Let us apply the function to 18191415 15 14 11 11 20 19 8 14 13
13149 10 1096 6 15 14398
Let A=0, B=1, C=2, D=3, E=4, F=5, G=6, H=7, 1=8, J=9, K=10, L=11
M=12, N=13, 0=14, P=15, Q=16, R=17, S=18, T=19, U=20, V=21
W=22, X=23, Y=24, Z=25
(c)
f(p) = (17p+ 22) mod 26
Let us apply the function to 1819 1415 15 14 11 11 20 19 8 1413
167017 17011247209
Let A=0, B=1, C=2, D=3, E=4, F=5, G=6, H=7, 1=8, J=9, K=10, L=11,
M=12, N=13, 0=14, P=15, Q=16, R=17, S=18, T=19, U=20, V=21
W=22, X=23, Y=24, Z=25
QHAR RABBYHCAJ