I need help..
could someone explain to me how to do this?

6x-4

6x - 4

is simply an expression. There's nothing to do, here. If you wish to "solve" something, you have to have an equation, like

6x -4 = 0

In this case, you solve the equation by figuring out what to add, subtract, multiply, or divide to isolate the x.

6x -4 +4 = 0+4 ' add 4 to both sides.

\frac {6x}{6} = \frac 46 ' divide both sides by 6

x = \frac 46 = \frac 23