see attachment..
btw, this is really basic stuff
you just pick an x and calculate the y. you need to pick at least 2 different x's to calculate 2 different y's and get 2 different points (x,y) through which you can draw a line.
for y = 3x + 4
lets say x = 0
y = 3 * 0 + 4 = 4
T1 (0,4)
then lets say x = -1
y = 3 * (-1) + 4 = -3 + 4 = 1
T2 (-1,1)
and you just draw a line through these 2 points..