The break-even point is the point at which net profit equals $0.
You can solve this using formulas taught in the books:
Break-Even in Units = Fixed Costs / Contribution Margin Per Unit
X = $1,200 / ($12-$4)
X = 150 Units
You can also just think logically. If they make $8 on every t-shirt, they'll have to sell 150 before they've made $1,200 and covered there costs. At that point, every additional t-shirt sold is $8 of profit, which is the contribution margin per unit.