step by step on how to solve this problem 2420/(570+r) = 2140/(570-r)

I would first of all divide both sides by 20 to get the numbers a bit smaller

121/(570+r) = 107(570-r)
now cross-multiply
60990 + 107r = 68970 - 121r
228r = 7980
r = 7980/228 = 35

or, look at it this way:
the LCD is (570+r)(570-r)
so in effect I just multiplied both sides of the equation by that LCD