input: Write a python function to generate a quadratic equation with random coefficients between -10 and 10.
  assert isinstance(generate_equation(), str)
  assert 'x^2' in generate_equation()
  assert 'x' in generate_equation()
output: import random
  
  def generate_equation():
      a = random.randint(-10, 10)
      b = random.randint(-10, 10)
      c = random.randint(-10, 10)
      
      # Ensure a is not zero to make it a quadratic equation
      while a == 0:
          a = random.randint(-10, 10)
      
      equation = f"{a}x^2"
      
      if b > 0:
          equation += f" + {b}x"
      elif b < 0:
          equation += f" - {-b}x"
      
      if c > 0:
          equation += f" + {c}"
      elif c < 0:
          equation += f" - {-c}"
      
      return equation