Verified answer

Solve for what? ... a ? , ... b ? , ... , a + b ? , ... or something else.

EDIT:

            (a + 3√2) • (3 – 4√2) = -18 + b√2

            3a – 4a√2 + 9√2 – 24 = -18 + b√2

            a(3 – 4√2) = -18 + b√2 – 9√2 + 24

            a = (b√2 + 6 – 9√2) ⁄ (3 – 4√2)

(a + 3√2) • (3 – 4√2) = -18 + b√2

3a – 4a√2 + 9√2 – 24 = -18 + b√2

a(3 – 4√2) + 9√2 – 6 = b√2

[  a(3 – 4√2) + 9√2 – 6  ] ⁄ √2 = b