Oh this is physics! We learned this too! I think u can solve this with proportion. I can't solve this because in my country we measure in kg, g, mg etc. Plus I'm foreign. But I will take a picture of my notes from school and try to translate something.
Unless there is more included in this problem then -6.61 is the slope. Slope is represented as m in y=mx+b. Simply substituting in the slop and y-intercept shows that m=-6.61 or the slope is -6.61.
