The system is in equilibrium, so the net force on each charge is zero.
For the charge +q:
$T_1 = \frac{k(q)(2q)}{d^2} + \frac{k(q)(4q)}{(2d)^2}$
$T_1 = \frac{2kq^2}{d^2} + \frac{4kq^2}{4d^2}$
$T_1 = \frac{2kq^2}{d^2} + \frac{kq^2}{d^2}$
$T_1 = \frac{3kq^2}{d^2}$
Show more…