I’m not 100% sure this is correct, because I’ve never done these before, but try 12, this is the answer I got from a reliable app that I usually use
x = 
isolate 5x by adding c to both sides of the equation
5x = k + c ( divide both sides by 5 )
x = 
Answer:
112
Step-by-step explanation:
Tried a whole bunch of odd sequences:
11 + 13 + 15 = 39
Nope too small.
21 + 23 + 25 = 69
Woah too big!
And after a few more trials found that:
17 + 19 + 21 = 51
Woohoo!