After factoring, you will get $(x-3)$ multiplied by a difference of two fractions. Rewrite $\frac12$ as a fraction with denominator $8$ so you can subtract them easily.

Once you have an equation of the form $\text{(fraction)}\cdot(x-3) = 15$, undo the fraction by multiplying both sides by its denominator, then solve for $x$.

In Desmos, type the expression (5/8)(x-3) - (1/2)(x-3) - 15 as a single line. This represents the left side minus the right side of the equation.

Look at the graph of that expression and find the x-intercept (where the graph crosses the x-axis). The x-value of this intercept is the solution to the equation.

Notice both terms on the left share the factor $(x-3)$:

Factor $(x-3)$:

Now you just need to simplify the expression in parentheses.

Rewrite $\frac12$ with denominator $8$:

So

Now the equation becomes

To undo the multiplication by $\frac18$, multiply both sides by $8$:

This simplifies to

Now add $3$ to both sides to isolate $x$:

So

Thus, the correct answer is 123.