Python (higher order function) - Don't understand how to approach this, any advice or hints please

I don't know where to start with this problem. Urgent help needed!!

def combine(f, op ,n):
    result = f(0)
    for i in range(n):
        result = op(result, f(i))
    return result

def smiley_sum(t):
    def f(x):

def op(x, y):

n  = ...

# Do not modify this return statement
return combine(f, op, n)

The question asks to define the two functions in terms of combine. However, I do not exactly understand what that means and how to implement that. From how I see it, the 'combine' function is the main function and we have to write the other two functions from scratch (without utilising combine in them).