sub in the y value which is 6:
2x+3*6=36
next you multiply 3*6 to start simplifying for x:
2x+18=36
after that you need to isolate the variable x by using inverse operations(subtract 18 from both sides:
2x+18-18=36-18
simplify that:
2x=18
do inverse operations again this time you need to use division:
2x/2=18/2
simplify for your answer:
x=9
:D