Well plug 0 in for x and solve for y and that is the vertex of your expression
Just a guess, but the angle inside the triangle that's not x or y is the remaining angle after subtracting the 30 degrees shown. That makes the non labeled angle inside the triangle 150 degrees. since all triangles are 180 degrees, this means x and y add up to 30. They look roughly the same, so i'd guess they're both 15 degrees.

he then turns around and grabs that money and sticks it for another 9 years,

add both amounts, and that's how much is for the whole 21 years.
Answer:
y=3x+1
Step-by-step explanation:
y=mx+c
Gradient is 3
Rise/run=3/1=3
Y intercept=1
y=3x+1