Take the sentence "y is 15% of x" and think of it as an equation:
y = .15 * x
Knowing that, we can put '.15x' in anywhere we see a 'y'.
Rather than 5870621 = x + y, we'll say
5870621 = x + .15x, so combine like terms for:
5870621 = 1.15x.
Do you see where to go from here?