Assuming simple interest, the interest after 12 months would be 0.04×210000=$8400.
The total payments after 12 months is 1400×12=$16800.
Simple interest means that the same amount (16800-8400=$8400) is deducted from what’s owing each year.
In T years therefore, $8400T would be paid off, so when 8400T=210000, the whole amount would be paid off, and T=210000/8400=25 years or 300 months.
(The answer is different for compound interest, when it would take only about 17 years 4 months to pay off the amount. A different formula is used.)