Given the expression
The objective is to solve for x, this means, isolate x on one side of the equal sign and all other terms on the other side of it.
To do so, you have to "pass" the term "35" from the left side of the equation to the right side. To pass a term from one side to the other you have to apply the opposite operation to both sides of it, this is so that the equality stays valid.
The number is positive (it's adding), so the opposite operation will be to subtract it:
Simplify the expression and you get that:
Now, note that the x-term is negative when you want to solve an equation for x, the variable must be positive, this means we have to turn "-x" into "x".
The minus also indicates that the coefficient multiplying x is "-1", usually when the variables are alone, there is a hidden coefficient multiplying them, for "x" the coefficient is "1" and for "-x" the coefficient is "-1".
To make x positive you need to cancel this multiplication, to do so you have to divide it by the same number. And to keep the equality valid, what is done to one side of the equation must be done to the other side, so divide both sides by -1
Once divided by -1 we determined that the value of x is 18