I'm not completely sure if I did this right because I haven't used this in a while, but let's say sodas are x and hot dogs are y. The first equation would be 3x+4y=15 and in order to solve for x or y you have isolate them to seperate sides because if there is not letter on the other side, it will just end up being zero... if that makes sense. subtract 3x so the equation is now 4y=15-3x and divide everything by 4, to make the equation y=3.75-3/4x. Do the same thing but subracting the 4y and dividing by 3 to get the x value. Now we're left with y=-3/4+3.75 and x=5-4/3y (or x=5-1 1/3y). remember that x=sodas and y=hotdogs. The equation we need to create has seven hot dogs and no sodas, so we simply need to take the equation for y and substitute y for 7 because we have 7 hot dogs and y is the symbol used for hot dogs. This equation would now be 7(-3/4x+3.75) because hotdog=y=-3/4x+3.75, so hotdog=-3/4+3.75 and we have 7 hot dogs so you just multiply that by 7
