Decrypt affine cipher given encryption key i have a ciphertext C=TLNJG formed using the equation
c=(7p+11) mod 27,c equivalent to numerical equivalent character of ciphertext and p the plaintext
how do i get the plaintext back?
for encryption:c=(7p+11) mod 27
for decryption:p=5(c-11) mod 27
let's say i encrypt the letter B(1) I would get

c=(7+11) mod 27
c=18 mod 27=18 (letter S)

but if I do decryption using the decryption equation of the letter S I will get the letter I instead of B.Why?

p=5(18-11) mod 27
p=35 mod 27=8(letter i)
Your formulas are wrong. If $$c \equiv 7p + 11 \mod{27}$$ then by applying the modular arithmetic function $$c - 11 \equiv 7 p  \mod{27}$$ and then $$(c - 11) \times 7^{-1} \equiv p  \mod{27}$$.

Therefore, the decryption function is: 
$$p \equiv (c - 11) \times 7^{-1} \mod{27}$$

For this we need to compute de multiplicative inverse of 7 modulo 27. Since they are coprime number, this inverse exists. And we can compute it using the Euler totient method like this:

$$7 ^{-1} \equiv 7 ^{\phi(27) -1}  \mod{27}$$

$$\phi(27) = 27 \times (1 - \frac{1}{3}) = 18$$
then 
$$ 7^{-1} \equiv 7^{17} \equiv 4  \mod{27}$$

Finally the decode function is :

$$p \equiv (c - 11) \times 4$$

For your example: 

$$(18 - 11) \times 4 \equiv 7 \times 4 \equiv 28 \equiv 1  \mod{27}$$