prostacyclins : imagine it as a ( cycle ) so it ( cycles the artery ) lol so it dilates it ( because u think that a circle is expanding ) , now , this lowers the pressure , likewise , if the platalets didnt aggregate , the blood will flow easily , so the pressure is still low ( e.g : its anti-aggregating )
now thromboxan is exactly the opposite ( think as it is a box well shut , so blood must struggle to get out of it thus tension is created & pressure is increased)
i hope u are not laughing at me 😀