the assignment is to multiply two strings of minimum 50 digits in each,and display result showing step by step multiplication

A high "cool story bro" factor on this one.. You omit information and you do not provide any attempts done by yourself or even own thoughts on how to fix it.

You could follow the primary-school like methods of multiplication e.g.

 18
 18
 -- *
144    // 8*8 = 64. Write 4 and remember 6. 8*1 = 8, Add remembered 6 = 14.
18     // Next number from the buttom. Start number 1 to the left. Rest is identical.
--- +
324    // Add the subresults to obtain the final results. Empty slots are 0.