Capacity of memory = 2^21 bit
So, Capacity of memory in Byte will be = 2^21/2^3 = 2^18 bytes
Word size of 4 Byte is given, from this we can calculate no. of words in memory = 2^18/ 2^2 = 2^16 words.
RAM chip size capacity is = 2K X 8 (means 8 bit word can be store in one cell of RAM), therefore, RAM capacity = 2^11 words.
No. of decoder line req. = 2^16/2^11 = 2^5 = 32