The frequency of radar is 780 MHz. The frequency of reflected wave from the airplane is increased by 2.6 kHz. The velocity of airplane is 

      

For radar shift in the frequency of reflected wave is given by $∆\mathrm{\nu } =\mathrm{\nu }\frac{2\mathrm{V}}{\mathrm{c}}$ where V is the velocity of the airplane.

 so $\mathrm{V} =\frac{\mathrm{c}∆\mathrm{\nu }}{2\mathrm{\nu }}$ = $\frac{3\times {10}^8\times 2.6\times {10}^3}{2\times 780\times {10}^6} =0.5\times {10}^3$ m/s