To write the equation of the line in slope-intercept form, we need to find the slope and the y-intercept of the line. We can use the slope formula to find the slope:
m = (y2 - y1) / (x2 - x1)
where (x1, y1) = (1, 2) and (x2, y2) = (-2, -1). Substituting these values, we get:
m = (-1 - 2) / (-2 - 1) = -1/3
Now we can use the point-slope form of the equation of a line to find the equation of the line:
y - y1 = m(x - x1)
where (x1, y1) = (1, 2) and m = -1/3. Substituting these values, we get:
y - 2 = (-1/3)(x - 1)
Simplifying this equation, we get:
y = (-1/3)x + 7/3
Therefore, the equation of the line in slope-intercept form that passes through the points (1, 2) and (-2, -1) is y = (-1/3)x + 7/3.