need help in this question

Question

Imagine that you are working as a roller coaster designer. You want to build a record-breaking coaster that
goes 70 m/sec at the bottom of the first hill. You estimate that the efficiency of the tracks and cars you are
using is 90%. How high must the first hill be?

Answer 1

from energy conservation:

Ei=Ef

mgh(potential)=0.5mv^2(kintic) 

but efficiency is 90% so

0.9*gh=0.5v^2

h=0.5(70)^2/(0.9*9.8)=?