[(5x)^7 * (5x)^4 * 25] / [(25x^2)^4 * 125x^2 = 100
[ 5^7 * x^7 * 5^4 * x^4 * 5^2] / [(5^2)^4 * (x^2)^4 * 5^3 * x^2] = 100
[ 5^(7+4+2) * x^(7+4)] / [5^(8+3) * x^(8+2)] = 100
[5^(13) * x^(11)] / [5^(11) * x^(10)] = 100
5^(13-11) * x^(11-10) = 100
5^2 * x = 100
x = 100/25 = 4