You want to find all the terms that have x as the variable.
This means: tx * 7 + 10 * -6x = 7tx - 60x, or (7t - 60)x.
Now we look at the coefficient in the question. The coefficient of x is -39.
7t - 60 = -39
7t = 21
t = 3 and bam that's your answer :)