a.) 3x + 4y
b.) 3(3x + 4y) = 9x + 12y
Use the distributive property here to multiply the three to all terms in the sum (they referred "sum" to the two terms that we added in step A).
c.) (9x + 12y) - (x - y) = 9x + 12y - x + y = 8x + 13y
Since for this step you're subtracting two different terms at once, you have to put them in parenthesis, and then put the negative sign right before the parenthesis (to show that it's subtracting). The negative sign acts as a -1, and anything multiplied to -1 becomes negative (or positive if the term was already negative). So -1 * x = -x, an -1 * -y = y. Thus, the next step after the distribution of the negative 1 as 9x + 12y - x + y.