Question

how did capitalist and mercantilist polucies of European states (Portugal,Netherlands/Dutch,Britain,and France) promote trade of slaves from africa?

Answer 1

They wanted faster and cheaper production of goods for a bigger profit, and slaves help with just that. 

Answer 2

Trade played a more central role in the mercantilist period of European history from 1500 to 1750 – sometimes referred to as early capitalism or trade capitalism – than in almost any other period.1 We must begin with the questions: When in human history did the first exchange of goods between Europe and the other four continents of Africa, Asia, America and Australia occur? Where are the origins of what one could describe as on-going exchange, as established economic relations to be found? These questions refer to an even larger global context because the global economic edifice changed fundamentally from "proto-globalization" to globalization .2 This process was primarily determined by Europe from the 15th to the 20th century. From the 16th century to 1914, trade within Europe at all times constituted the most significant portion of global trade, and the volume of that trade grew disproportionately quickly during the early modern period and into the modern period.3 National markets became increasingly interconnected, driven by numerous innovations in the areas of infrastructure, transportation , energy supply, and – not least – institutions (rules, constitutions, division of labour, currency standards, etc.). The transition from individual production to mass production  and the convergence of prices of goods and materials made transactions considerably easier, thereby accelerating integration.

▲1

Starting in the late Middle Ages at the latest and continuing at least into the 19th century, Europe dominated most developments in international trade. From the end of the 19th century, North America  began to exert a stronger influence on the global economy.4 Around the beginning of the 21st century, the Asian states – most notably China – gained influence and the USA became financially dependent on its East Asian creditors, while China seems to become the engine of growth of the current century.