The measurement of arc DB equals 74.57°. Step-by-step explanation: To clarify the resolution method: A secant intersects a circle in precisely two points, while a tangent intersects at just one point. When a tangent and a secant intersect outside of a circle, the formed angle's measurement is precisely half of the positive difference between the measures of the intercepted arcs. Now, applying this to the issue where secant CE intersects circle A at points D and E, while tangent CB touches circle A at B, forming angle ECB. As such, m∠ECB = 1/2 (m arc EB - m arc DB). Given m arc EB as 96° and m arc DB given as 25x + 21, along with m∠ECB as 5x, we establish the equation 5x = 1/2[96 - (25x + 21)]. After rearranging and solving, we find x = 75/35, simplifying to 15/7. Next, substituting x back into the arc DB formula gives m arc DB = 25(15/7) + 21 = 522/7 yields approximately 74.57°.
