Getting the equation for a linear line

Question

1 = a + 4.5 x 2

or

1 = a + 9

or

a = -8

thus

y = -8 + 4.5x

so

i know how to get the slope but not sure how a was found

 

Answer 1

Second equation above

1 = a + 9

-8 = a       subtract 9 from both sides