Answer:
Minimum value of function
is 63 occurs at point (3,6).
Step-by-step explanation:
To minimize :

Subject to constraints:

Eq (1) is in blue in figure attached and region satisfying (1) is on left of blue line
Eq (2) is in green in figure attached and region satisfying (2) is below the green line
Considering
, corresponding coordinates point to draw line are (0,9) and (9,0).
Eq (3) makes line in orange in figure attached and region satisfying (3) is above the orange line
Feasible region is in triangle ABC with common points A(0,9), B(3,9) and C(3,6)
Now calculate the value of function to be minimized at each of these points.

at A(0,9)

at B(3,9)

at C(3,6)

Minimum value of function
is 63 occurs at point C (3,6).
I know you don't wanna do the work or don't know how but just get the m a t h w a y a-p-p and type the equations in and it'll tell you what x is
[12(.3)+x(1)]/(12+x)=.4
3.6+x=4.8+.4x
.6x=1.2
x=2 qts
(though your radiator would still be short a quart for doing so :) )