The question is ambiguous as presented. Let's take each interpretation and look for a solution. First let's subtract 420 from each side:
-2/315x2+210=4/3x. I'm going to assume x2 means x^2 (x squared).
Interpretation 1
(-2/315)x^2+210=(4/3)x. (2/315)x^2+(4/3)x-210=0 becomes x^2/315+2x/3-105=0
This can be multiplied by 315 to give x^2+210x-105*315=0. But 315-105=210 which is the central term, so we can factorise:
(x-105)(x+315)=0, so the roots are x=105 and -315.
Interpretation 2
-2/(315x^2)+210=4/(3x). This interpretation is similar to Interpretation 1 but x is replaced by 1/x, so the solution is x=1/105 and -1/315.