4000(1.5x)=25000 divide both sides by 4000
1.5x=6.25 divide both sides by 1.5
x=25/6
x=4 1/6
But that is pretty simplistic, I suspect that you actually meant this was an exponential function of the form:
4000(1.5^x)=25000 divide both sides by 4000
1.5^x=6.25 take the natural log of both sides
xln(1.5)=ln(6.25) divide both sides by ln(1.5)
x=ln(6.25)/ln(1.5)
x≈4.52 (to nearest hundredth)