Answer:
b=18.76
Explanation:
Let the line that splits the big triangle into two small triangles be x.
Let the unmarked side of the big triangle be y.
Consider the big triangle. By Pythagorus:
(6+16)²=b²+y²
484=b²+y²
y²=484-b²
Consider the small left triangle. By Pythagorus:
y²=6²+x²
y²=36+x²
x²=y²-36
Consider the small right triangle. By Pythagorus:
b²=16²+x²
b²=256+x²
b²=256+x²
b²=256+y²-36
b²=220+y²
b²=220+484-b²
2b²=704
b²=352
b=18.7616630393
b=18.76