If we stretch the length of the wire, it will increase by three. How will its resistance change?

The volume of the wire will be constant. The wire is a cylinder.

V = S * h, where S is the base area (cross section), h (l) is the height (length).

The wire length has increased 3 times:

S * l = S1 * l1.

S * l = S1 * 3l.

S1 = S * l / 3l = S / 3.

Initial resistance:

R = ρ * l / S.

Resistance after pulling:

R1 = ρ * l1 / S1.

R1 / R = (ρ * l1 / S1) / (ρ * l / S) = (ρ * 3l / S / 3) / (ρ * l / S) = 3 / 1/3 = 3 * 3 = 9.

Answer: The resistance of the wire will increase 9 times.