Write the equation of the graph.

Use the slope intercept form 

y=mx+b

m=slope
b=y intercept

slope= Δy/Δx

the slope is -2 so m=-2
y=-2x+b

the y intercept is -5 so b=-5

finally your equation is y=-2x-5

Find the slope of the graph, it will help. (Rise/Run)

the law of definite proportions states samples of a compound will always contain the same proportion of elements by mass. in other words, a given compound will always contain the same elements in the same proportions by mass.the law of multiple proportions states when two elements combine with each other to form more than one compound, then the weights of one element that combine with a fixed weight of the other are in a ratio of small whole numbers.