Complex sentence: On April 15, "The Wall Street Journal" determine that the Trump campaign had reported spending more than $500,000 in payments to companies owned by Trump, amounting to 6% of the 6.5 million that the campaign reported spending in the first quarter of 2017.
Split into simpler sentences: On April 15 The Wall Street Journal determined that the Trump campaign had reported spending more than $500,000 in payments to companies which Trump owns. This amounts to 6% of the 6.5 million that the campaign reported spending in the first quarter of 2017.