Question 798503
We are going to take two partial derivatives of the equation 
w(x, t) = L + (2 / (x +Lt +a)
rewite the equation as
w(x, t) = L + 2*(x +Lt +a)-1
note that L and a are arbitrary constants
@w/@t = 2*-1*L*(x +Lt +a)-2 = -2L/(x +Lt +a)^2
note that for the above partial derivative x is constant
@w/@x = 2*-1*(x +Lt +a)^-2 = -2 / (x +Lt +a)^2
note that for the above partial derivative t is constant
@w/@x ----> @^2w/@x^2 = @w/@x (-2 * (x +Lt +a)^-2 = 4 / (x +Lt +a)^3
now we can calculate
@^2w/@x^2 + w*@w/@X
= 4 / (x +Lt +a)^3 + (L +2 / (x +Lt +a)) * (-2 / (x +Lt +a)^2)
= 4 / (x +Lt +a)^3 + -2L / (x +Lt +a)^2 -4 / (x +Lt +a)^3
= -2L / (x +Lt +a)^2
= @w/@t, as required.