Divide the second equation by the first giving 2 = (7^y)/(5^y)
then 2 = (7/5)^y or 2 = (1.4)^y
Solve for y = ln 2 / ln 1.4
Substitute into the first equation for y
4^(x) = 5 ^(ln 2/ ln 1.4)
x ln 4 = (ln2/ln1.4) (ln 5); change ln 4 to ln 2^2 = 2 ln 2
x = (ln 1.4) (ln 5)/(ln 2)