It is extremely important to understand the words of the given problem. There are numerous information's of immense importance that are already given in the question. Based on those information's the answer to the question can be easily deduced.
Let us assume the unknown number = x
Then
14x - 34 = 78
14x = 78 + 34
14x = 112
x = 112/14
= 8
So the unknown number in the given question is 8. I hope the procedure is clear to you and you got the answer that you wanted.