5. As shown in Figure 3, each face of the cube is written with a natural number, and the sum of the two numbers on opposite faces is equal. If the number opposite to 10 is a prime number $a$, the number opposite to 12 is a prime number $b$, and the number opposite to 15 is a prime number $c$, then $a^{2}+b^{2}+c^{2}-a b-a c-b c=$ $\qquad$ .
5.19.

Given that $10+a=12+b=15+c$. Therefore, $c=2$. Then $a=7, b=5$.