Depends if it's (5/7x)^2 or if it's (5/7)x^2.
Also depends if it's (4/21)x or 4/(21x).
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Most probably you're asking about (5/7)x^2 = (4/21)x.
Multiply 5/7 * 3/3 to get a common denominator of (15/21)x^2 = (4/21)x.
Multiply both sides by 21 to get 15x^2 = 4x
Subtract 4x from both sides to get 15x^2 - 4x = 0
Factor out x and 15 to get 15x(x-4/15) = 0
x cannot = 0 because you can't divide by 0 in the original equation, so x = 4/15.
Cheers