Let's solve a similar question: Solve the proportion $$\dfrac{2.7}{5.2} = \dfrac{\square}{26}$$

Let $x$ represent the missing number. Cross-multiplying we get $$26 \times 2.7 = 5.2 \times x,$$ if we then divide both sides by $5.2$ we get that $$x = 26\times 2.7 \div 5.2 = 13.5,$$ so the missing number is $13.5$.