MATLAB: How to find x of y by y of x on a equation

i need to find x of y by y of x

for example if i have y = 2*x

i need to find x so the answer is x = (1/2)*y

how to do that using Matlab

syms x y
eqn = y == 2*x
solve(eqn, x)

B=xlsread('cp_lambda.csv');
x1=B(:,1);
y1=B(:,2);
figure(2)
plot(x1,y1)
x2 = 0.1;
y2 = spline(x1,y1,x2);
hold on
plot(x2,y2,'or')
hold off

x=0:0.1:10;
y1=cos(x);
y2=sin(x);
x1=2.234
y11=interp1(x,y1,x1)
Y21=interp1(x,y2,x1)