The information given places no restriction at all on the value that x can have. It can be any number - integer, rational, irrational or complex - of any magnitude.
2x3 + x2 + 15 This is a 3rd order polynomial in 'x'. It's numerical value kinda depends on the value of 'x'.
negative 15
30 + 5x = 15 + 10x 30 = 15 + 5x 15 = 5x x = 3
That depends entirely on the value of x.
That heavily depends on the value of 'x'.
There is not enough information to solve for x. x+5x+15 must equal something (is it 0?) in order to solve for x.
what the solution to this equation x+15=28 If you mean: x+15 = 28 then the value of x works out as 13
-15
3(2)-9=15 x=2
find x+x+x when x=15 15+15+15=45
15 (x = 135)
That depends on what the value of x is.