Grid[{{"h, km",0,1.525,3.05,4.575,6.1},{"ρ",1,0.8617,0.7385,0.6292,0.5328}},Frame-> All]

info = {{0,1},{1.525,0.8617},{3.05,0.7385},{4.575,0.6292},{6.1,0.5328}}

plot = ListPlot[info,PlotStyle->{ Blue},AxesLabel->{"Височина","Плътност"},PlotLegends->{"Данни"}]

f1[x_] =Fit[info,{1,x},x]

plot1=Plot[f1[x],{x,0,6.2},PlotStyle->{Dashing[0.04],Thick,Black},AxesLabel->{"Височина","Плътност"},PlotLegends->{"Приближение до (x^1)"},PlotRange->{0.5,1.1}]

f2[x_] =Fit[info,{1,x,x^2},x]

plot2=Plot[f2[x],{x,0,6.2},PlotStyle->Red,AxesLabel->{"Височина","Плътност"},PlotLegends->{"Приближение до (x^2)"},PlotRange->{0.5,1.1}]

plotobstho=Show[plot,plot1,plot2]

Limit[((f2[x+Δx]-f2[x])/(Δx)),Δx->0]

skorostf2[x_] =-0.094822+0.006001 x

plotskorost=Plot[skorostf2[x],{x,1900,2000},PlotStyle->{Blue,Thick},AxesLabel->{"Височина","Скорост на изменение"}]

GraphicsRow[{plotskorost,plotobstho}]

plutnost$4=f2[4]