input: Write a python function to calculate the total price of items including tax.
  assert calculate_total_price(10, 0.05, 5) == 52.5
  assert calculate_total_price(20, 0.1, 3) == 66.0
  assert calculate_total_price(15, 0.07, 4) == 64.2
output: def calculate_total_price(price, tax_rate, quantity):
      total_price = price * quantity
      tax_amount = total_price * tax_rate
      return total_price + tax_amount