First, we will find the slope
Pick two points(A, B) and locate its cordinates
I pick point A(-3 3)
POINT B(0,0)
Using the slope formula;
slope = Y₂ - Y₁ / x₂ - x₁
x₁ = -3 y₁=3 x₂=0 y₂=0
substituting the value into the formula;
slope = 0-3 /0+3
slope = -3/3
slope = -1
Intercept is zero from the graph
Substituting
m= -1 and b=0 into;
y=mx + b
y= -x + 0
y=-x