simplify brackets
8 - 2a - 7 = a + 40
simplify 8 - 2a - 7 to -2a + 1
-2a + 1 = a + 40
add 2a to both sides
1 = a + 40 + 2a
simplify a + 40 + 2a to 3a + 40
1 = 3a + 40
subtract 40 from both sides
1 - 40 = 3a
simplify 1 - 40 o -39
-39 = 3a
divide both sides by 3
-39/3 = a
simplify 39/3 to 13
-13 = a
switch sides
a = -13