Genetics problems at this level almost always come down to Mendel's laws and a Punnett square. The maths is trivial. The setup is where mistakes happen.
Monohybrid: Aa × Aa gives 1 AA : 2 Aa : 1 aa. Phenotypes: 3 dominant : 1 recessive. Memorise this.
Dihybrid: AaBb × AaBb gives 9 A_B_ : 3 A_bb : 3 aaB_ : 1 aabb — the famous 9 : 3 : 3 : 1, from two genes assorting independently.
Test cross: an unknown-genotype dominant crossed with a homozygous recessive. If any recessive offspring appear, the unknown is heterozygous.
X-linked recessive traits appear more often in males. If a trait skips a generation and appears in a grandson but not the mother, X-linked recessive is very likely.
Always write out the parental genotypes explicitly before drawing the Punnett square. The mistakes at this step ruin the whole problem.