F(x)=−14x+12 for x=3?

Pls , the question is in the picture!

Nick and nora are married and have three children in college. they have an adjusted gross income of 47,400. if their standard deduction is 12,600 itemized deductions are 14,200 and they get an exemption of 4000 for each adult and each dependent what is their taxable income

How do you solving the solution of each system using elimination?