plz help(This is the whole problem it says "one part remaining")

He invested two unknown amount that add to $1200.
Let the amount invested at 9% be x.
Let the amount invested at 3% be y.

The amounts add to $1200.
x + y = 1200           First Equation

The profit from the 9% part is 0.09x.
The profit from the 3% part is 0.03y.
The two profits add to $78.
0.09x + 0.03y = 78        Second Equation

We have two equations in two unknowns, so we can solve this problem as a system of equations.

x + y = 1200
0.09x + 0.03y = 78

Multiply the first equation by -0.09 and add to the second equation.

-0.09x - 0.09y = -108
 0.09x + 0.03y =   78
----------------------------
             -0.06y = -30

Divide both sides by -0.06

y = 500

Now we substitute 500 into y in the first equation.

x + y = 1200

x + 500 = 1200

x = 700

Answer: The amount invested at 9% is $700