The subtraction property of equality allows us to subtract the same number from both sides of an equation preserving its validity.
For example, the equation x + 2 = 3 can be subtracted 1 and obtain x + 1 = 2
We are given the equation:
10x + 6 = 21
We need to subtract a number in such a way that the 21 turns into a 15.
That number is 6, thus, subtracting 6 on both sides:
10x = 15
The missing expression is 10x as underlined above.