помогите плиз x/3+x/4=-21

х/3+х/4=-21
4х/4*3+3х/3*4=-21
4х/12+3х/12=-21
7х/12=-21
7х=-21*12
7х=-252
х=-252/7
х=-36

-36/3+(-36/4)=-36/3-36/4=-12-9=-21