Question

If the point (a,3) lies on the graph of the equation 5x + y = 8, then a=

A) 1
B) -1
C) 7

Answer 1

`5a+3=8\\ 5a=5\\ a=1\Rightarrow A`

Answer 2

`(a,3) \\ x=a \\ y=3 \\ \Downarrow \hbox{plug the values into the equation} \\ 5a+3=8 \ \ \ |-3 \\ 5a=5 \ \ \ |/ 5 \\ \boxed{a=1} \Leftarrow A`