Respuesta :

Answer:

a = -3

Step-by-step explanation:

3a + 4a = -21

3a + 4a = 7a

7a = -21     Divide both side with 7

-21 / 7 = -3

a = -3

To make it true...

3(-3) + 4(-3) =

-9 + (-12) = -21

-21 = -21