First of all, you’ve a fraction in your equation, accompanied by a value that needs to be added. This, you can not directly shift (+b) to the other side.
As you’ve got two values in brackets being multiplied by one quarters too, you multiply them to obtain:
[8b-80]
———— + b = 70
4
To add b into our fraction, we must make a common denominator, and so we multiply b over 1 with 4 on both the numerator & denominator, obtaining:
[8b-80+4b]
—————— = 70
4
From here, it is simple. You shift the 4 to the other side, multiplying it with 70 to get 280. Shift the negative 8 to the other side, making it positive while doing so, and add that to 280, obtaining 288. Now you only have 12b=288, so you divide both sides of the equation with 12, leaving you with 24.