I think I'll come back to this later. I started with how I would approach it.

log(28)/log(35) = log(4*7)/log(5*7) = [log(4) + log(7)] / [log(5) + log(7)] = [2 log(2)/log(7) + 1] / [log(5)/log(7) + 1]

a = log(49) / log(14) = 2 log(7) / log(2*7) = 2 log(7) / [log(2) + log(7)] = 2 / [log(2)/log(7) + 1]. Thus log(2)/log(7) = 2/a - 1

b = log(5) / log(14) = log(5) / log(7*2) = log(5) / [log(7) + log(2)] = 1 / [log(7)/log(5) + log(2)/log(5)]. Thus log(5)/log(7) = 1/b - log(2)/log(5)

log(2)/log(5) = log(2)/log(7) * log(7)/log(5) = (2/a - 1) * (1 / (1/b - log(2)/log(5))

From here, isolate log(2)/log(5). (breakpoint)