im not sure how to explain it but--
for example.. the first one
1) y + 7 = 12
first, you subtract 7 from both sides
so-- you'll subtract 7-7 on the left and 12-7 from the right
now, you have y by itself since 7-7 = 0
y = 12-7, or y = 5
also its important to note that when the equation is like 2) and uses subtraction (x-4) instead of addition (y +7)-- then you add to both sides
so for 2)-- you'd add 4 to both sides for 4 to cancel out to equal 0
your goal is always to isolate the variable on one side