The probability of afailure each time that the coin is flipped is 1/2 = 0.5.
If that is the question, the solution was pretty simple and the answer is 0.5.
But, it seems that they could be asking for the probability of not getting a head in 70 flips.
If this is the case, then the probability of failure is (1/2)^70 = 8.47*10^-22.
Then it is necessary to have the full context, but what I have explained should give the answer. Let me know, please.