The formula for the frequency in a stringed instrument : f=12 Tmf = \frac{1}{2} \sqrt{\; \frac{T}{m}}f=21mT
f = frequency; T = tension in the string; m = mass per unit lenth of the string.
f1_{1}1 = 14 = 12 Tm\; \frac{1}{2} \sqrt{\; \frac{T}{m}}21mT.
When T is quadrupled, we have
f2_{2}2 = new frequency = 12 4Tm\; \frac{1}{2} \sqrt{\; \frac{4T}{m}}21m4T
= 2( 12 Tm\; \frac{1}{2} \sqrt{\; \frac{T}{m}}21mT)
= 2 f1_{1}1
= 2 x 14
= 28 Hz
