Step one
Write the equation with a slope of - 4/5
Notice that -4/5 = - 0.8
So the equation so far is
y = -0.8x + b
Step Two
Use the point to find b
When x = - 2 then y = - 5
-5 = -0.8(-2) + b
-5 = 1.6 + b Subtract 1.6 from both sides.
-5 - 1.6 = b
-6.6 = b
Step 3
Write the complete equation
y = - 0.8x - 6.6
Comment
The fact that you are given a fraction for the slope makes me think that the answer should reflect a fraction. If you have choices, you should give them.