Does anyone knows how to do this i don’t understand it.

Between the two, the difference in x is -1 and the difference in y is 3. 
Y's difference is rise, and x's difference is run. 
Divide rise over run to get the slope. 
3 / -1 = -3 
So the slope is -3, eliminating 2 and 4. 
The y-intercept that fits the graph is 7, not -7. 
That means the correct equation is C. 
Hope this helps!