The formula for slope-intercept form is: y = mx + b
m = the slope of the line
b = the y-coordinate of the y-intercept, (0,b)
In this case, the slope is 3, since you go up 3 and to the right 1 to get from the point at (1,1) to the point at (2,4). 3/1 = 3
The line crosses the y-axis at (0,-2), so your b-value is -2.
The equation is y = 3x - 2.
In general, lines that increase (go uphill) from left-to-right have a positive slope. Lines that decrease (go downhill) from left-to-right have a negative slope.