Factor each term separately down to prime numbers:
-30a = -1*2*3*5*a
-100 = -1*2*2*5*5
The factors they have in common are shown in orange - so factor them out. The factors in black are what's left of the 30a and 100 after you remove the common factors:
-30-100 = (-1*2*3*5*a) + (-1*2*2*5*5)
= (-1*2*5)(3*a + 2*5) =
(-10)(3a+10)
BTW, -30a-100 is an algebraic expression, not an equation. An equation always has an equal sign (=).