יהושע א:ז פירוש: רש״י

Just be very strong and courageous. This was said in reference to the Torah, as He states: ‘To preserve, to act in accordance with all the laws of the Torah.’

תַּשְּׁכִּיל—You will be successful.¹³People consider that a successful person is a smart person. That is why the word תַּשְׂכִּיל—is associated with the word תַּצְלִיחַ. (Metsudas Tzion).