Three particles, charge $$q_1 = +10 \ \mu C, q_2 = -20 \ \mu C,$$ and $$q_3 =+30 \ \mu C, $$ are positioned at the vertices of an isosceles triangle as shown in the above figure. If $$a=10 \ cm$$ and $$b= 6.0 \ cm,$$ how much work must an external agent do to exchange the positions of
(a) $$q_1$$ and $$q_3$$ and, instead,
(b)$$q_1$$ and $$q_2$$?