Given that,
Spring constant =k
Displacement =x
Let x be the initial displacement of the block. Then the force on the block are −k1x and −k2x
So, the force is
F=−k1x−k2x
F=−(k1x+k2x)
Now, the acceleration is
F=ma
a=Fm
a=−(k1+k2)xm
Hence, the acceleration is −(k1+k2)xm