The equation is y = 3.5x
The equation can be written in linear form as : 
- y = bx + c 
- b = slope ; c = intercept 
b = Rise / Run 
From the graph:
Slope, b = (7 - 0) / (2 - 0) = 3.5 
From the graph, the Intercept, which is the value of y when x = 0 is 0.
Hence, the equation becomes :