As per the NCERT textbook, the question is "a moving electron has 5x10^-25 J of kinetic energy. What is the de Broglie wavelength", (and not 5 x 10 j)

Here it is given, 

mass of electron = 9.1 x 10^-31 kg

Kinetic energy i.e. 1 / 2 mv² = 5 × 10^-25 J

That is, 1 / 2 x (9.1× 10-31) v² = 5 x 10^-25

 v² = 5 x 10^-25 x 2 / 9.1× 10^-31

So, v = 1.048 x 10³ m/s

Now, Planck’s constant h = 6.62 x 10^-34 Js = 6.62 x 10^-34 kg m² s^-1

Wavelength, λ  = ?          

λ = h / m v 

λ =  (6.62 × 10^-34 kg m² s^-1)/(9.1 × 10^-31 kg x 1.048 x 10³ m sec⁻¹)

λ = 6.9 x 10^-7 m.