This assumes that B is not a function of s! It also assumes that C is a circle centered about a line source of current. Please make sure to post the whole question! (For those that didn't catch that, this is a well known problem about the magnetic field around a line current. The problem uses Ampere's Law.)
∫CBB⋅Bds=∫CBB2ds=∫CBds=B∫Cds as I've mentioned elsewhere.
Now, your contour C is a circle of radius r, and C=2πr, thus ds=dC=2πdr.
I haven't left much, but can you finish on your own?
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