Front area: 12/x*3=36/x

Front-end cost: 400*36/x= 14400/x

Side area: 3*x*2=6x

Side cost: 150*6x=900x.

House price: y = 14400/x+900x+5800 (0

(2) y'=- 14400/x？ +900

Single increment interval: y'>0

- 14400/x？ +900 & gt； 0

x？ & gt 14400/900

X & lt-4 or x> IV

Single subtraction interval: -4

When y'=0, y gets the minimum value.

x？ = 14400/900

x= 4

When x=4, y takes the minimum value: y =14400/4+900 * 4+5800 = 23800 yuan.