When typing down equations with trigonometric functions, you should use the virtual keyboard. Otherwise the solver considers them as variables:

In order to solve the equation, we will plug 't = cos(x)' and solve for 't':

6t^2 - t - 1 = 0

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Click here to see the step by step solution for 't'

........

t1 = 1/2

t2 = -1/3

Now, go back to solving:

cos(x) = 1/2

x = pi/3

x = 60 degrees

cos (x) = -1/3

x = 109.47 degrees approximately