help needed june 2009 unit 3b phy

[JustAnotherPerson22]

i really can't solve part (E) in question 8
can't seem to figure out how we got rid of μ
help will be appreciated

 

[alex]

gradient= 1/4μf^2
gradient=0.16 m^2N^-1
μ=6.2x10^-4 kgm^-1
solve for f