Question 18696
When you are given the slope (m) and the y-intercept (b), you can start by writing the equation in the slope-intercept form: {{{y = mx + b}}} then convert it to standard form: {{{Ax + By = C}}}. 
N.B. I've capitalized the A, B, and C in the standard form because the B here is not the b that represents the y-intercept.

You are given the y-intercept, b = 8 and the slope, m = 3, so you can write:

{{{y = 3x + 8}}} as the slope-intercept form. Now convert to the standard form.

{{{3x - y = -8}}}

For the second problem, you are given the slope, {{{m = 3/4}}} and the y-intercept, {{{b = -2}}}.  First, write the slope-intercept form of the equation: {{{y = mx + b}}}

{{{y = (3/4)x + (-2)}}} Simplify.
{{{y = (3/4)x - 2}}} Now convert to standard form: {{{Ax + By = C}}}

{{{(3/4)x - y = 2}}} To clear the fractional x-coefficient, simply multiply through by the denominator, 4.

{{{3x - 4y = 8}}} Standard form sans (without) fractional x-coefficient.

On the third problem, you are correct that the the point (0, -6) is the y-intercept, so you can go through the same process as for the first two problems.

Start with the slope-intercept form.

{{{y = (3/5)x - 6}}} Now convert to standard form:

{{{(3/5)x - y = 6}}} and the clear the fraction, multiply through by the denominator, 5.

{{{3x - 5y = 30}}} Standard form sans fractional x-coefficient.