Let's start by multiplying everything in the paranthesis by ln

(ln)e^0.4x + (ln)e^-0.4x - (ln)15

just as a square root and something squared will cancel each other,
so will ln and e^
so let's start canceling

(ln)e^0.4x + (ln)e^-0.4x - (ln)15

that leaves us with

0.4x + (-0.4x) - 2.71 = 0

0.4x + (-0.4x) = 2.71

0 x = 2.71

= no solution