or, 3x + 5x - 7x = 10 - 17 ( take all the variables in one side and all the constants in another side)
or, 8x - 7x = -7
or, x = -7
you can check the answer by putting x = -7 into the equation. after putting the value of x if the Right hand side and left hand side of the equation become same, then your answer is correct.
Sometimes there is no apparent way to factor it by hand /(the factors are decimals or imaginary). To try and factor these problems is nearly impossible in your head.