x^3n+ 1=(x^n)^3+ 1=3^3+ 1=28

2.x^m= 1/5,x^n=3

x^(3m+n)+ 1=(x^m)^3*x^n+ 1=( 1/5)^3*3+ 1=3/ 125+ 1= 128/ 125

3.2^m+4^m+8^m=2^ 18

This problem would be easy if it was x, and it seems that there is no integer solution after adding it.

x+x^2+x^3=2^ 18

solve an equation