-1 is a solution, and, to put it another way,

2^x + 3^x + 6^x - x^2 = 0,   for x = -1

The function on the LHS has derivative

ln(2)2^x + ln(3)3^x+ln(6)6^x - 2x

which, at x=-1, equals approximately  3.011404264707352, which is positive, and in fact is everywhere positive, as can be seen by graphing, so x=-1 is the only solution.