This doesn't make any sense because you didn't give a value to cos like you did to sin.
Let me guess the actual question is tan(x)*cos(x) write this in term of sin.
Now from trigonometry identities, we know that tan(x)=sin(x)/cos(x)
So the expression can be written as sin(x)/cos(x)*cos(x)
which results in sin(x) as the answer, if I guessed the question wrong, sorry and please tell me in the comments what the correct question is, because this question doesn't seem complete :)