If you know the distance of an earthquake epicenter from 3 seismic stations, how can you find the exact location of the epicenter of the earthquake?

Measure the distance between the first P wave and the first S wave.

http://www.geo.mtu.edu/UPSeis/locating.html

white blood cells or wbc s are mainly 2 types-

1.agranulocytes

2.granulocytes

the monocytes or macrophages of agranulocytes , dendritic cells which are accessory leucocytes, and b cells which is one type of lymphocytes   identify the antigens present on the pathogen's surface. these three cells are called antigen presenting cells(apc) which are phagocytes and destroy the antigens by secreting antitoxins. besides this leukocyte is always plays vital role in the immunity of the body.