.
It is very elementary and basic skill that every student must develop to the level to solve such problems on his (or her)
own AUTOMATICALLY without asking assistance from outside.
Similar to as you answer "4" when you are asked "How many is 2*2 ?"
To help you to develop such skills, a special lesson was created in this site
- Equation for a straight line having a given slope and passing through a given point
Find there a complete explanation on how to do it and learn the subject once and for all.
Do not miss other associated lessons that cover more wide subject.
You will find the links at the end of the referred lesson.
Consider these lessons as your textbook, handbook, tutorials and (free of charge) home teacher.