But before that discussion got underway, Trump expressed her thoughts about the tragic church shooting in Texas.
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"God bless the people of Sutherland Springs, TX," she tweeted. "Our country's hearts are breaking for the victims & their families. We love & are with you!"
Trump, who serves as an advisor to her father President Donald Trump, and Mnuchin discussed the administration's tax plan before a crowd of more than 300 people.
Trump said she's particularly concerned about working families.
"Tax relief is about creating a culture of work and dignity in this country," she said, "and giving our working parents greater economic stability, so they can make the best choices for their own families."
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Mnuchin said the administration will reduce the business tax rate and overhaul the tax code for the first time since Reagan was in office.
"Great to have joined @IvankaTrump @Reagan_Library to discuss #TaxReform," he tweeted afterward. "We'll cut taxes for families & make US businesses more competitive."
Mnuchin will discuss tax reform with Southern California business leaders.