6x+7y=4x+4y Complete the missing value in the solution to the equation

How do i do this? i don’t understand

You were told that the amount of time lapsed between consecutive trades on the new york stock exchange followed a normal distribution with a mean of 15 seconds. you were also told that the probability that the time lapsed between two consecutive trades to fall between 16 to 17 seconds was 13%. the probability that the time lapsed between two consecutive trades would fall below 13 seconds was 7%. what is the probability that the time lapsed between two consecutive trades will be between 14 and 15 seconds?

Can someone me with #s 8, 9, and 11. with just one of the three also works. prove using only trig identities.