How to plug in the solution of an equation into an expression?

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In the code below, how would I plug in the solution for x in Ln[18] into Ln[19]? (I should get Out[19] -5*a+10)

In[16] var('a,x')
In[17] eq1 = x+a==0
In[18] solve([eq1],x)
In[19] 5*x+10

PS: 1. Is there a way to declare 'a' a constant rather than a variable?