Answer: 10
The arc length of the semicircle is
a+(a+1)+(a+2)+(a+3)+(a+4)=5a+10
As x = 42, this means a+4 is 42/180 of the arc length of the circle, 5a+10.
So,
(42/180)(5a+10)=a+4
42(5a+10)=180(a+4) [multiply both sides by 180]
210a+420=180a+720 [distributive property]
30a+420=720 [subtract 180a from both sides]
30a=300 [subtract 420 from both sides]
a=10 [divide both sides by 30]