Brett took a loan of $35,000 at a rate of 6%. after 3 years, what is the interest rate he had to pay

if this is a simple interest problem, and you meant to ask "What is the amount of interest he had to pay?"

I = Prt
I = 35000 * .06 * 3
I = $6,300

Brett would be paying $2100 for the first year ($175), assuming he had payed off the interest which accrued monthly. Over a three year period, Brett would pay $6300 in interest payments, again assuming he had payed the interested which accrued monthly. By the wording of your question (which im assuming was a mistake), technically, he would still be paying 6% unless the interest rate fluctuated over the 3 years. However, if you meant to say "What is the total amount of interest he would have payed" the answer would be $6300, assuming he made all his monthly payments on time and did not pay anything towards the principal amount.

A=P(1+R/N)^nt