f ( x ) = 435 + 1200 x
They had $435 to start with ( b = 435 ) and 1200 is the slope ( m ) in the formula:
y = m x + b
The coefficient 1200 means the money they get per one week ( $1,200 ).
We can also solve the equation:
10,000 = 435 + 1,200 x
1,200 x = 9,565
x = 9,565 : 1,200 = 7.97 ≈ 8 weeks