再试一下:try again, try one more time.
再想一下:think again, think one more time.
差不多: close, almost, but not qutie.
So in your scenario, you could say: 差不多,但是不完全对。请再试一下,可以吗?
Which means: close, but not exactly/completely correct. please try again, alright/is that ok?