Assuming “bigger” refers to volume.
Let R be the radius of a sphere.
The surface area of a sphere is A = 4*pi*R^2.
The volume of a sphere is V = pi*R^3*4/3.
The volume of the new planet is Vnew = 2.4*Vearth.
This means that
pi*Rnew^3*4/3 = 2.4*pi*Rearth^3*4/3,
which implies
Rnew = (2.4)^(1/3)Rearth.
The area of the new planet is therefore
Anew = 4*pi*(2.4)^(2/3)*Rearth^2 = (2.4)^(2/3)*Aearth.
The land area on the new planet is then
Lnew = 0.31*(2.4)^(2/3)*Aearth.
Using Aearth = 196,900,000 sq. miles we then have
Lnew = 0.31*(2.4)^(2/3)*196,900,000 = 109,416,185 sq. miles.