#### Answer

$x + y - 3 = 0$

#### Work Step by Step

1. First we will find the slope of line using two intercepts (0,a) and
(a, 0)
m = ($y_{2}$ - $y_{1}$)/($x_{2}$ - $x_{1}$)
m = (0 - a) / (a - 0)
m = -1
2. Now, again using the same equation and given point on line
( 1,2), we will find the value of a
($y_{2}$ - $y_{1}$)/($x_{2}$ - $x_{1}$) = m
(2-a)/ (1- 0) = -1
a = 3
3. So, equation of line will be:
$\frac{x}{3}$ + $\frac{x}{3}$ = 1
3x + 3y = 9
3x +3y -9 = 0
x + y - 3 = 0