Can't find the answer for when they arrived at the same time

Abbey purchased a house using a fixed rate mortgage. the annual interest rate is 4.2% compounded monthly for 20 years. the mortgage amount is $235,000. what is the monthly payment on the mortgage? this isn't a multiple choice question have to show my work

What is the solution to the equation? k/6.4=8.7 2.3 5.568 15.1 55.68

Asap! i'm not good math and i'm lost.