People who are really good at math can handle complex math problems. The rest of us are all set with simple stuff like the times tables. Use complex to refer to something that is not simple.
Something complex has two or more related parts, or is not simple in its structure. As a noun, a complex is a group of units, often a group of buildings. The adjective is stressed on the second syllable, but the noun is stressed on the first syllable. Complex is from Latin complexus, from complecti "to encircle or embrace."