Transpose the formula for ‘A’ and find the value of A when k1 = k2 = 0.2

Transpose the formula for ‘A’ and find the value of A when
k1 = k2 = 0.2

k3 = 0.5 

x1 = 40 x 10-3

x2 = 125 x 10-3

x3 = 60 x 10-3

Rt = 500