How do i solve this ? i have the answer but i don’t get it

Using the pythagoras theorem 
r^2 = 12^2 + (18-r)^2
r^2 = 144 + 324 - 36r + r^2
36r = 144 + 324 = 468
r =  468 / 36 = 13 cm Answer

sin 1/2 < AOB = 12 /13
1/2 < AOB =  67.38
< AOB = 134.8 Answer

Area of major sector OAPB

ARea of the circle = pi r^2 = 144pi

so area of mafor sector OAPB =  (225/2 / 360) * 144pi =  90.08 pi  or
283 cm^2 to nearest whole number

“exterior angles of triangles. an exterior (or external) angle is the angle between one side of a triangle and the extension of an adjacent side. try this drag the orange dots on any vertex to reshape the triangle.” look to get to the answer i explained what it means and other things i hope it you .