What is the surface area of the prism?
length: 15mm
width: 3mm
height: 4mm

The answer is:  " 234 mm² " .

Explanation:

This would refer to a rectangular prism that has 6 (SIX) sides.

The formula for the surface area for a rectangular prism is:

Surface area:  2ab + 2bc + 2ac ;

In which, Given the length, width, and height of the prism; 

a = length of one of the three dimensions; 
b = length of one of the three dimensions (one of the remaining two                                                            dimensions not used to be represented by "a"); 
c = length of one of the three dimensions (the remaining dimension that is not                                            already represented by "a" or "b" ; 

Given:

Length:  15 mm ;
Width:      3 mm ;
Height:     4 mm ;

Let:  a = 15 mm ; 
        b =   3 mm ;
        c =   4 mm ;

Since the formula for the surface area of a rectangular prism is:

   Surface area:  2ab + 2bc + 2ac ;

           We can solve for the surface area by plugging in our given values:

Surface area: = 2(15mm)*(3mm)  +  2(3mm)*(4mm) + 2(15mm)*(4mm) ;
    
                       =   90 mm²    +   24 mm²   +   120 mm²  ;
 
                       =   234 mm² .