input: Write a python function to calculate the sum of squares of two numbers.
  assert sum_of_squares(5, 9) == 106
  assert sum_of_squares(3, 4) == 25
  assert sum_of_squares(0, 0) == 0
  assert sum_of_squares(-3, 3) == 18
output: def sum_of_squares(a, b):
      return a**2 + b**2