Additional info:(Lumda λ =1.42), i need to solve for m and n..
At the place where you got stuck, the equation has the form:
A - ½ = B
where A and B are complicated expressions. Just square both sides:
(A - ½)^2= B^2
A^2 - 2(A)(½) + (½)^2 = B^2
Then can simplify and solve for n.