Question 425052
Set y = 15.4 and solve for x:
{{{15.4 = 2.6*sqrt(x) + 9}}}
Subtract 9 from RHS, divide by 2.6 and square both sides:
(6.4/2.6)^2 = x = 6.059 years after 1980.
Therefore, the year was 1986.