a) \(x-7+15-\left(3+15\right)\)
\(=x-7+15-18\)
\(=x-10\)
b) \(2a-a-\left(a+1999\right)+2000\)
\(=a-a-1999+2000\)
\(=1\)
a, x - 7 + 15 - (3 + 15)
= x - 7 + 15 - 18
= x - 7 + (-3)
= x - 10
b, 2 x a - (a + 1999) + 2000
= 2 x a - a - 1999 + 2000
= 2 x a - a - (-1)
= 2 x a - a + 1
= a - 1