Answer is x = 4
Explanation :-
As B is midpoint of SR :- SB = BR and if. SR = 6x then SB = 3x
AC parallel SR ( MID POINT THEOREM )
Hence AC parallel SB
Similarly , AB parallel CS (MID POINT THEOREM)
So ABCS is a parallelogram
And we know that in parallelogram opposite sides are equal so AC = SB
So now we have AC = 5x - 8 and SB = 3x
So, 5x - 8 = 3x
5x - 3x = 8
2x = 8
x = 4