There is a formula for your problem

(y-y1) = m(x-x1)

where:
y = your y variable
y1 = your known y-value
m = gradient
x = your x variable
x1 = your known x-value

lets take one of the co-ordinates: (3,6)
you say the the gradient is 1/2.

Therefore, by subsituting in the value to my formula, I get:

(y-y1) = m(x-x1)

y = y
y1 = 6
m = 1/2
x = x
x1 = 3

(y-6) = 1/2 * (x-3)

y-6 = (x/2) - 3/2

y = (x/2) - 3/2 + 6

y = (x/2) + 9/2

y = 0.5x + 4.5