make a good drawing and use Pythagorean Theorem

h = the distance that separates the parallel sides, h>0

h^2 = 10^2 - ((1/2)(21 - 9))^2

by solving we find:

h = 8

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d = the length of each diagonal, d>0

d^2 = 8^2 + ((21 - (1/2)(21 - 9))^2

d = 17

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let A and B represent the angles

sin A = h/10

sin A = 8/10

A = 53.1 degrees

sin B = (1/2)(21 - 9)/10

sin B = 0.6

B = 36.9 +90

B = 126.9 degrees

another way to find B is to use that the sum of the angles of the isosceles trapezoid is 360:

2(A + B) = 360

2(53.1 + B) = 360

by solving we find:

B = 126.9 degrees

click here to see the step by step solution of the equation: