find the value of x or y so that the line through the points has the given slope. (12,y) and (15,-8); slope: 2/3
(y+8)/(15-12) = 2/3
(y+8)/3 = 2/3
y+8 = 2
y = -6
Also, you could have noted that a slope of 2/3 means y increases by 2 when x increases by 3.
Since x went from 12 to 15 (an increase of 3), y would have to increase by 2 from -8, or -6.