We have to "cancel" x's in the expansion
This will happen when we have the term
C (8,4) (2x)^4 * ( k/x)^4 = 700000 simplify
70 * 2^4 * k^4 = 700000 divide out 70
70 * 16 * k^4 = 10000
16 * k^4 = 10000
k^4 = 10000/16 take the positive 4th root
10/2 = k = 5
