4200 : 70 - 34 * 70 + 678

Let's calculate the expression step by step:

1. First, we divide 4200 by 70:
$$4200 : 70 = 60$$

2. Next, we multiply 34 by 70:
$$34 * 70 = 2380$$

3. Now, we substitute the results into the expression:
$$60 - 2380 + 678$$

4. Perform the subtraction and addition from left to right:
$$60 - 2380 = -2320$$
$$-2320 + 678 = -1642$$

So, the final answer is:

Answer: -1642