63 initially
Tonya is running a marathon she ran 12 miles she had 51 miles left now many miles is the whole marathon
By the law of cosines, the angle

satisfies

which reduces to

which converts to approximately 77 degrees (this can be done my multiplying by 180 and dividing by pi).
Since ABCD is a parallelogram, the opposite sides will be parallel and equal,

Consider that AC acts as a transversal to the parallel lines AB and CD, so we can write,

So by the ASA criteria, the triangle AED is congruent to the triangle CEB,
Then the corresponding parts of the triangles will be equal,

Hence Proved.