Question

Please check

Answer 1

\[d = 3a + 3b\]

Solve for b. Lets start by isolating.
Subtract 3a from both sides

\[d - 3a = 3b\]

Divide the entire equation by 3, to isolate b

\[\frac{ d - 3a }{ 3 } = b\]

Answer 2

You are correct

Answer 3

In general, solving for a variable is just getting it by itself on one side of an equation.
You do that by adding, subtracting, and multiplying (and possibly more as you get older) both sides of the equation.