We are given the expression
5a + 10 - 3a + X = 4a +10
Where X is the expression needed to make the equation true (the blue line).
Simplifying:
2a + 10 + X = 4a + 10
Subtraction 2a + 10:
X = 4a + 10 - 2a - 10
Simplifying:
X = 2a
Thus, the blue line must contain the expression 2a
