The question wants me to calculate the \$V_3\$ value,and the solution is as below 

Solution:

\$V_3=(\frac{10}{2}+5)(2//3)=12V\$
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However,here is my thinking,by the way,my answer is not the same as solution's,but i don't know where am i wrong,can anyone tell me where am i wrong?

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\$V_3=I_3\times R_1\$,and \$I_3=I_1+I_2=5+(\frac{10}{R_1+R_2})=5+(\frac{10}{2+3})=5+2=7\$,

so \$V_3=I_3\times R_1=7 \times 3=21V\$

Where am i wrong?i think my calculation should be correct,but the my answer is wrong!