Multiply a string by a number!
Jelly, 6 5 4 bytes

²Ɠxm


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How it works

²Ɠxm  Main link. Argument: n (integer)

²     Yield n².
 Ɠ    Read and eval one line of input. This yields a string s.
  x   Repeat the characters of s in-place, each one n² times.
   m  Takes each |n|-th character of the result, starting with the first if n &gt; 0, 
      the last if n &lt; 0.