summing op-amp: 2 voltages in parallel
There is no current flowing into the inverting (negative) input, because the input impedance of an ideal op-amp is infinite, therefore the current flowing through the feedback 1k\$\Omega\$ resistor (let's call it \$i_3\$) is the sum of the two currents flowing through the two other resistors (\$i_1\$ and \$i_2\$). The inverting input is a virtual ground, as the op-amp with negative feedback always tries to make the voltage between its inputs zero. Let's name the two inputs (the 200Hz and the 20Hz ones) \$u_1\$ and \$u_2\$, and the output voltage \$u_3\$. You can now use a nodal equation for the non-inverting input (let's say every current is flowing to the right):

\$i_1+i_2=i_3\$

\${u_1-0 \over {1k\Omega}} + {u_2-0 \over {1k\Omega}}={0-u_3 \over {1k\Omega}}\$

Now multiply the equation with 1k\$\Omega\$:

\$u_1+u_2=-u_3\$

There you have the sum of the two input voltages on the output (multiplied by minus one).