Substitute 6 for x in the equation, sx = 2sx−1+sx−2 and work carefully from there. s6 = 2s6−1+s6−2 , which simplifies to s6 = 2s5+s4. However, you don’t know s5 or s4. Use the equation to find these missing terms. s4 = 2s3+s2. and the problem tells you s2 and s3 are equal to 2. s4 = (2 × 2) + 2, which is 6. Now you need to find s5. Using the equation, you get s5 = (2 × 6) + 2, which is 14. Now that you know s5 and s4, go back to your original equation, s6 = 2s5+s4, and s6 = (2 × 14) + 6, which is 34.