Explanation:
the distance from a vertex of the quadrilateral to the touching points on the two sides emanating from the vertex is the same.
and that applies to each of the 4 vertices.
so, AB splits into the distance a (from A to the circle touching point on AB) and b (from the touching point on AB to B).
b = AB - a
in the same way, BC splits into b (B to circle touching point on BC) = AB - a, and c (circle touching point on BC to C).
c = BC - (AB - a) = BC - AB + a
CD splits into c (C to circle touching point on CD) = BC - AB + a, and d (circle touching point on CD to D).
d = CD - (BC - AB + a) = CD - BC + AB - a
DA now splits into d (D to circle touching point on DA) = CD - BC + AB - a, and into ... a (the circle touching point on DA to A).
a = DA - (CD - BC + AB - a) = DA - CD + BC - AB + a
0 = DA - CD + BC - AB
DA = CD - BC + AB = 8 - 5 + 4 = 7