Answer:
7^x = 2^(3x - 1)
x ln 7 = (3x - 1) ln 2
x ln 7 = 3x ln 2 - ln 2
x(ln 7 - 3 ln 2) = -ln 2
x = (ln 2)/(3 ln 2 - ln 7) = about 5.191
5^(-7x) = 7^(1 + 3x)
-7x ln 5 = (1 + 3x) ln 7
-7x ln 5 = ln 7 + 3x ln 7
x(-3 ln 7 - 7 ln 5) = ln 7
x = -(ln 7)/(3 ln 7 + 7 ln 5) = about -.114