Question

I have to go for dinner with my family. I won't be able to come for the party (complex sentence)

Answer 1

A complex sentence is a sentence which has at least one independent and one dependent clause.

The answer for your question ---->


I won't be able to come for the party as I have to go for dinner with my family. 
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The underlined part of the sentence is dependent clause and the part which isn't underlined is the independent clause. 

Answer 2

i wont be able to come for the party because i have to go with my family for dinner