



 
Hi Patrick, I am assuming this is a circular arch. If the length of a circular arc is $a$ units and the angle subtended by the arc, at the center of the circle is $t^o$ than \[a = 2 \pi \; r \frac{t}{360}.\] Thus you need to find $r$ the radius of the circle and the measure $t$ of the angle subtended by the arc. In my diagram of your arc $C$ is the center of the circle, $A$ and $B$ are the ends of the arc and $P$ is its apex. $AC = r, PD = 30$ inches and $DA = 24$ inches. Thus $ADC$ is a right triangle with sides of length $AD = 24$ inches and $DC = r  20$ inches. Its hypotenuse is of length $r$ inches. Use Pythagoras Theorem to solve for $r.$ Use a trig function to find the measure of angle $DCA.$ I hope this helps, Penny 



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