input: Write a function to calculate the square root of a user-defined positive number. If the given number is negative, return an error message. 
  assert my_function(100)==10.0
  assert my_function(9)==3.0
  assert my_function(25)==5.0
  assert my_function(-1)=="Error: Cannot compute square root of negative number"
output: def my_function(num):
      if num < 0:
          return "Error: Cannot compute square root of negative number"
      return num ** 0.5