x=√(13x-42).
I believe (13x-42) is under the radicand. If so, square both sides:
x² =[√(13x-42)]²
x² = 13x - 42 or
x² - 13x + 42 = 0. Solve this quadratic equation:
x' =[-b + √(b²-4.a.c)]/2a and x" =[-b - √(b²-4.a.c)]/2a
x' = [13+√13² - 4(1)(42)]/2 and x" = [13-√13² - 4(1)(42)]/2
x' = (13+√1)/2 and x" = (13-√1)/2
x' = 7 and x" = 6
