The problem here is: Too many words.
If you boil the problem down to the bare facts, the question is:
"$210 is 15% of what number ?"
I'm pretty sure you can take it from there,
but here's the rest of it anyway:
$210 = 15% of (March amount)
$210 = 0.15 x (March amount)
Divide each side by 0.15 : $1400 = March amount .
Check:
1400 + 210 = 1610
1.15 x 1400 = 1610 yay!