How did you get the 3a-2.1/2 =8 explain

This is the substitution step. We were given that the value of $b$ is $\frac{1}{2}$.
So, in the expression $3a - 2b$, we replace the variable $b$ with its numerical value, $\frac{1}{2}$.

When you have $2b$, it means $2 \times b$.
So, when $b = \frac{1}{2}$, then $2b$ becomes $2 \times \frac{1}{2}$.

Therefore, $3a - 2b = 8$ becomes:

$$3a - 2 \times \frac{1}{2} = 8$$

or

$$3a - 2\left(\frac{1}{2}\right) = 8$$

Both notations mean the same thing: 2 multiplied by $\frac{1}{2}$.

Then, we perform the multiplication:

$$2 \times \frac{1}{2} = \frac{2}{1} \times \frac{1}{2} = \frac{2 \times 1}{1 \times 2} = \frac{2}{2} = 1$$

So the equation simplifies to:

$$3a - 1 = 8$$

This is how we get to that step.