$3 x+8-x=42$

Step 1 ：Combine the x terms: $3x - x = 2x$

Step 2 ：Write the equation with the combined x terms: $2x + 8 = 42$

Step 3 ：Subtract 8 from both sides: $2x = 34$

Step 4 ：Divide both sides by 2: $x = 17$

Step 5 ：\(\boxed{17}\)