Math problem help, For which year the average life expectancy be?

Question

According to one mathematical model, the average life expectancy for American men born in 1900 was 55 years. Life expectancy increased by about 0.2 years for each birth year after 1900.
If this trend continues, for which birth year will the average life expectancy be 85 years?

I know the answer is 2050; I need help setting it up? What am I making x be?

Answer 1

Increase of 0.2 is the slope. (1900 , 55) is a point on the line.
y - 55 = 0.2 (x - 1900)
y = 0.2(x - 1900) + 55

85 = 0.2 (x - 1900) + 55
30 = 0.2 (x - 1900)
30 = 0.2x - 380
410 = 0.2x
2050 = x

Answer 2

There is an increase of 30 years.
30/.2 = x
x = 150
1900 + x = 2050