4^(-x ) - 7*2^(-x) - 6 = 0
2^ ( - x) = t ==>
t^2 - 7t - 6 = 0
D = 49 + 24 = 73
t1 = ( 7 + √73)/2 ≈ 7,77
t2 = (7 - √73)/2 ≈ - 0,77 (не удовлетворяет условию)
2^( - x) = ( 7 + √73)/2
log2 2^(-x) = log2 ( 7 + √73)/2
- x = log2 ( 7 + √73)/2
x = - log(2, ( 7 + √73)/2) ≈ - 2,9583