assuming you're looking for "x"
one may note that you never asked anything btw, anyhow
not to bore you to death with the "inscribed quadrilateral conjecture", but you can search in google if you want, I can also include a quick proof of it
but in short it boils down to, a quadrilateral or 4-sided polygon, inscribed in a circle, has opposite angles that are "supplementary angles"
what the dickens does all that mean?
well, it means that A + C = 180 and B + D = 180
since those are the pair of opposite angles in this one
hmm so we know what A and B and C are in x-terms, what about D?
well, a 4-sided polygons has a total sum of all internal angles to be 180(n-2) or 180(4-2) or 360 in short
so.. A + B + C + D = 360
that means
(10x) + (6x+3) + (4x+12) + D = 360
A B C D
20x + 15 + D = 360
that means
D = 360 - 15 - 2x <--- whatever that is
so... now we know what D is in x-terms
well, from the "inscribed quadrilateral conjecture" we know that B + D = 180
thus (6x+3) + (360 - 15 - 2x) = 180
solve for "x"