3x - 5 + x = 10 + 5 <---------- This is your initial equation
4x - 5 = 15 <-------------------- Step 1: The first thing you want to is simplify both sides of the equation (combine like terms). As shown in the initial equation, 3x + x is 4x and 10 + 5 is 15.
4x = 20 <------------------------- Step 2: Isolate the x term on one side, and the terms without x on the other. To do this, you must add 5 to both sides of the equation.
x = 5 <----------------------------- Step 3 (final step): Simplify. In order to fully solve, and calculate what x equals, you must divide both sides by 4 to get the x by itself.
**In most questions like these, the ultimate goal is to find out what x equals.
Hope this helps!