You have that the given triangles are congruent. Then, you can consider that sides BC and DE are equal, that is:
BC = DE
Furthermore, you have tha the previous sides are given by the following algebraic expressions:
BC = 2x - 1
DE = x + 9
In order to find the value of x you equal the previous expressions for BC and DE (becuase these sides are equal):
2x - 1 = x + 9 subtract x both sides
2x - x - 1 = 9 add 1 both sides
2x - x = 9 + 1 simplify
x = 10
Hence, x = 10