X = 7
y = 10
x + y = 700
7x + 10y = 6400
10x + 10y = 7000
(-) 7x + 10y = 6400
______________
3x = 600
x = 200
200 + y = 700
y = 500
(-3x+4) + (5x + 12) = 2x +16
The answer is D.
Hope it helps!
I would use the pythagorean theorem to find the lengths of each side. a² + b² = c².
Side AB is one we're looking for. If you make other right triangle with that same side you can see that one length is 4 and the other is 3. So, 4² + 3² = c² → 25 = c² → 5 = c. Side AB is length 5.
Side AC is another. Do the same with that side and you get that one length is 4 and the other is 3. (This is the same one as above) so side AC is length 5.
Side BC is the last one. One of the lengths is 1 and the other is 1 → 1² + 1² = c² → 2 = c² → 1.414213562 = c so side BC is approximately length 1.41.
Add each length up and you get a perimeter of roughly 11.4
Let's take a look at a perfect square trinomial.

We can distribute this expression to get

.
An important relation to recognize here is that the constant is the square of half of the x value.
Now, if we wanted to add a number to

to make a perfect square trinomial, we could have the value of x and then square it.
Half of 16 is 8, and 8 squared is