If abc is congruent to cbd, then what is the value of x?

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?

I’m stuck on this equation, anyone got the answer?

Which property was used to simplify the expression? 3c+9 +4c=3c+4c+9